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GEOLOGY 


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GEOLOGY. 
By Tuomas C. CHAMBERLIN and ROLLIN D. SALISBURY, 
Professors in the University of Chicago. (American 
Science Series) 3 vols. 8vo. 
Vol. I. Geological Processes and Their Results. 
Vols. II and III. Earth History. (Not sold separately.) 


A COLLEGE TEXT-BOOK OF GEOLOGY 
By Tuomas C, CHAMBERLIN and ROLLIN D. SALISBURY, 
(American Science Series.) 8vo. 


INTRODUCTORY GEOLOGY. A TEXT-BOOK FOR 
COLLEGES. By Tuomas C. CHAMBERLIN and ROLLIN 
D. SALISBURY. (American Science Series.) 12mo. 


PHYSIOGRAPHY 
By RoLuin D. SALISBURY. (American Science Series.) 8vo. 
The same. Briefer Course. 12mo. 
The same. Elementary Course. 12mo. 


ELEMENTS OF GEOGRAPHY 
By Rotiin D. SALISBURY, HARLAN H. BARROwsS and 
WALTER S. Tower, of the Department of Geography, 
The University of Chicago. (American Science Series.) 
12mo. 


MODERN GEOGRAPHY 
By Rotimn D. SALISBURY, HARLAN H. BARROws and 
WALTER S. Tower, of the Department of Geography, 
The University of Chicago. (American Science Series.) 
I2mo. 


HENRY HOLT AND COMPANY, PUBLISHERS 
New YORK AND CHICAGO 





AMERICAN SCIENCE SERIES. 


INTRODUCTORY 
GEOLOGY 


pe eeXT-BOOK FOR COLLEGES 


BY 


THOMAS C. CHAMBERLIN 


AND 


ROO IN sD: SALISBURY 


Heads of the Departments of Colbey and Saas 
The University of Chicago 






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tay iy) 
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NEW YORK 


HENRY HOLT AND COMPANY 
1924 





Coryricnt, 1914 
BY 
HENRY HOLT AND COMPANY 


The Lakeside Press 
R. R. DONNELLEY & SONS COMPANY 
CHICAGO 





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PREFACE 


This volume is an abbreviation and simplification of COLLEGE 
GEOLOGY, published five years ago. Many technical details have 
been omitted, but the general purpose and scope of the volume is 
not altered fundamentally. It is intended to present an outline 
of the essential features of geology with as few technicalities as 
the nature of the subject permits. Part I deals with geological 
processes, and with the materials on which they operate, while the 
y> theme of Part II is historical geology. The effort has been to treat 
“ these topics in such a way as to give the student not merely an 
- understanding of the subject, but also an understanding of the 
means by which the present status of the science has been reached. 
N The theoretical and interpretative elements which enter into 
“the general conceptions of geology have been used freely, because 

| they are regarded as an essential part of the evolution of the science, 

2 since they often help to clear and complete conceptions and to stim- 

“ ulate thought. The aim has been, however, to characterize hypo- 
"y thetical elements as such, and to avoid confusing the interpreta- 

\ tions based on hypothesis with the statements of fact and estab- 
.~ lished doctrines. 


~~) 


>< In many cases the topics discussed will be found to be pre- 
. sented in ways differing widely from those which have become 
S familiar. In some cases, fundamentally new conceptions of familiar 
[subjects are involved; in others, topics not usually discussed in 
2y text-books are stated with some fullness; and in still others, the 
emphasis is laid on points which have not commonly been brought 
YMnto prominence. Whether the authors have been wise in depart- 

_ ding to this extent from beaten paths, the users of the volume must 
} decide. 

Note University of Chicago, 


February, 1914. 


HEDRICK ' 


Jans 





CONTENTS 


PAR ie! 


THE MATERIALS. OF THE EARTH AND PROCESSES 


CHAPTER 
A 


II. 


III. 


IV. 


WHICH AFFECT THEM 


PRELIMINARY OUTLINE 


THE EARTH IN THE SOLAR SYSTEM 
THE GRAND DIVISIONS OF THE EARTH 


GEOLOGIC WORK OF THE ATMOSPHERE 


MECHANICAL WoRK 

CHEMICAL WorK ; 
THE ATMOSPHERE AS A CONDITIONING “AGENCY ; 
SUMMARY 


WORK OF GROUND (UNDERGROUND) WATER 


GENERAL FACTS ‘ 

WorK OF GROUND- WATER 

ORE DEPOsITs . ‘ 
SUMMARY . 
SPRINGS AND ARTESIAN W ELLS : 


WORK OF RUNNING WATER 


EROSIVE WorRK 

ANALYSIS OF EROSION 

CONDITIONS AFFECTING RATE OF EROSION 

RATE OF DEGRADATION 

FEATURES RESULTING FROM SPECIAL, ConDITIONS OF ERo- 
SION . oe SAC OY etre 1 ee: 

EFFECTS OF UNEQUAL “HARDNESS 

THE EROSION OF FOLDs . 

ADJUSTMENT OF STREAMS TO Rock STRUCTURES - 

INFLUENCE OF JOINTS ON EROSION 

EFFECT OF CHANGES OF LEVEL . 

AGGRADATIONAL WoRK OF RUNNING WATER . 

ALLUVIAL TERRACES epee. ae 


V. WORK OF SNOW AND ICE 


IcE IN GENERAL 

GLACIERS 

THE STRUCTURE OF GLACIER IcE 
MotTION OF GLACIER ICE 

THE WorkK OF GLACIERS 
GLACIO-FLUVIAL WoRK 
ICEBERGS 


Vil 


PAGE 


i) 


Vili 


CHAPTER 
VI. 


XIII. 


XIV. 


CONTENTS 


WORK OF THE OCEAN 


GENERAL FACTS 

PROCESSES IN OPERATION IN “THE SEA 
MOVEMENTS OF SEA-WATER . 
DEPOSITS ON THE OCEAN-BED 


LAKES 
OUTLINE OF THEIR WorRK AND HISTORY 


MOVEMENTS AND DEFORMATIONS OF THE EARTH’S 
BODY (DIASTROPHISM ) 
MINUTE AND Raprp (SErsMIc) MOVEMENTS, EARTHQUAKES 
SECULAR MOVEMENTS sl an. ype arn 
VULCANISM 
INTRUSIONS . 


EXTRUSIONS 
THE CAUSE OF VuLc ANISM 


MATERIALS OF THE EARTH AND THEIR ARRANGE- 
MENT 


IGNEOUS Rocks 

SEDIMENTATION AND SEDIME NT. ARY Rocks. 

INTERNAL CHANGES IN IGNEOUS AND SEDIMENTARY Rocks; 
METAMORPHISM : , 

VARIOUS CLASSIFIC. \TIONS AND NoME NCLATURES ~ 


PAROLE 


HISTORICAL GEOLOGY 


THE ORIGIN OF THE EARTH 
HYPOTHESES 


STAGES OF THE EARTH’S HISTORY PRIOR TO) THe 
KNOWN ERAS 


STAGES UNDER LAPLACIAN HYPOTHESIS 
STAGES UNDER PLANETESIMAL HYPOTHESIS 


THE ARCHEOZOIC ERA 


GENERAL CONCEPTIONS 

GENERAL CHARACTERISTICS OF ARCHEAN Rocks — 
DISTRIBUTION OF ARCHEAN ROCKS . 
THEORETICAL CONSIDERATIONS : : 
GENERAL TABLE OF GEOLOGIC TIME DIvIsIONsS ‘ 


THE PROTEROZOIC ERA 


FORMATIONS AND PHySICAL HISTORY 

THE PROTEROZOIC OF THE LAKE SUPERIOR REGION ; 

GENERAL CONSIDERATIONS RELATING TO LAKE SUPERIOR 
PROTEROZOIC : , 

PROTEROZOIC OUTSIDE “LAKE ‘SUPERIOR REGION , 

LirE DurRING PROTEROZOIC ERA 

CLIMATE OF PROTEROZOIC ERA . 


PAGE 


167 
170 
173 
189 


201 


206 
427 


228 
229 
241 


246 
205 


285 
297 


299 


397 
310 


314 
317 
320 
323 
323 


325 
331 


337 
340 
342 
342 


CONTENTS 1X 
THE PALEOZOIC ERA 


CHAPTER PAGE 
XV. THE CAMBRIAN PERIOD 

PORMATIONG ANO WL HYSICAL) HISTORY: Ur Se eaerho wk ow. (34g 

Bee TH APASEUROAN To) ig Wa eo ce ce yO NE ey te ny 38S 


XVI. THE ORDOVICIAN (LOWER SILURIAN) PERIOD 


Pewee TIONS AND AE HYSICAL, HISTORY 2. Gl. pes a 4 1967 
LIFE . a 


XVII. THE SILURIAN (UPPER SILURIAN) PERIOD 


FORMATIONS AND PHysIcAL HisToRyY . . . .:. . . 388 

Pre ee Seca RTM a eg a aad 
XVIII. THE DEVONIAN PERIOD 

FORMATIONS AND PHysICAL HISTORY . .. . . . . 402 

ema Pl eee UR i gl aR 


XIX. THE MISSISSIPPIAN (EARLY CARBONIFEROUS) PE- 


RIOD 
FORMATIONS AND PHysICAL HISTORY Uaets AN 2) Pea ataG 
See te SS Un ron fey tele ehh. ke 1432 
XX. THE PENNSYLVANIAN (UPPER CARBONIFEROUS) PE- 
RIOD 
FORMATIONS AND PHysicAL History . .. . . . . 44I 
SERIE WAY ost at ae ERA rm ts ye fers erage 


XXI. THE PERMIAN PERIOD 


FORMATIONS. AND PHYSICAL. History a CG eee my 
RT Ge OE i a a a 
PROBLEMS OF THE PERMIAN . 4 


THE MESOZOIC ERA 


XXII. THE TRIASSIC PERIOD 


PORMATIONSFAND) LHYSICALMELISTORY. (07) So. we, 48d 
ce Re ROM ae eh Les eh eee Ta MM, ee aK lee! AOR 


XXIII. THE JURASSIC PERIOD 


FORMATIONS AND PHysICAL History .., .. . . 502 
Mei ts ee we Pence ey Nia is) in. 5) gt VSOF 
XXIV. THE COMANCHEAN (LOWER CRETACEOUS) PERIOD 
RURMATIONS AND PHYSICAL HISTORW My) ge eh. 1 88 
Pera ene, ars Ty ee hee I Me WM yet ge a S28 


XXV. THI CRETACEOUS PERIOD 


FORMATIONS AND PuysicAL History . . . . . . « 532 
Pra nT Sa | ae Re og! che seis flee (SAO 


XXVII. 


XXVIII. 


XXIX. 


XXX. 


CONTENTS 


THE CENOZOIC ERA 


THE EOCENE AND OLIGOCENE PERIODS 


FORMATIONS AND PuHysicAL HISTOR 
Dire Se ee a a a eS 
OLIGOCENE FORMATIONS . 
OLIGOCENE LIFE 


THE MIOCENE PERIOD 
FORMATIONS AND PHysICAL HISTORY 
LIFE . re eyes : 
THE PLIOCENE“ PERIOU 
FORMATIONS AND PuHysicAL HISTORY 
LIFE . : ee ee A 
THE PLEISTOCENE OR GLACIAL PERIOD 
FORMATIONS AND PHYSICAL HISTORY 
CAUSE OF GLACIAL CLIMATE 
Lire. ee oa 
THE HUMAN OR PRESENT PERIOD 
FORMATIONS. 
LIFE . 


APPENDIX 


REFERENCE TABLE OF THE PRINCIPAL GROUPS OF PLANTS . 
REFERENCE TABLE OF THE PRINCIPA?, GROUPS OF ANIMALS 


VII. 
VIII. 


XVI. 


Winn Obe PIA TES 


FACES PAGE 


DUNES IN CONTOUR 

STREAMS DISAPPEARING IN SAND, GRAVEL, ETC., IN AN ARID REGION 
YOUTHFUL VALLEYS, SHORE OF LAKE MICHIGAN 

THE WIDENING OF A RIVER VALLEY BY MEANDERING OF THE STREAM 


YOUTHFUL VALLEYS IN A REGION OF SLIGHT RELIEF AND OF GREAT 
RELIEF 


Fic 1. ToPpoGRAPHIC MATURITY 


Fic 2. IRREGULARITIES OF COAST DEVELOPED BY EROSION AND 
DEPOSITION 


TopoGRAPHIC OLD AGE 
CUSHETUNK AND RounD MownrtaIns, N. J. . 
Fic. 1. ENTRENCHED MEANDERS. CONODOGUINET CREEK, PA. 


Fic. 2. SECTION OF THE CALIFORNIA COAST AT OCEANSIDE, 
SHOWING CHANGES OF LEVEL OF THE LAND 


ALLUVIAL FAN AT THE BASE OF MOUNTAINS. CUCAMONGA, CAL. 
THE ALLUVIAL PLAIN OF THE MIsSsouRI AND BiG Sioux; S. DAK. 
GLACIERS OF GLACIER PEAK, WASH. 

GLACIERS AND CIRQUES OF THE BIGHORN MOUNTAINS 

AN ILt-DRAINED PLAIN OF GLACIAL DRIFT; SOUTHERN WISCONSIN 
Fic. 1. SHORE-LINE OF MARTHAS VINEYARD, Mass. 

See ILAND-TIED ISLAND 2 2.50 soos 6 #8 oe 
PmeeUPPeR END OF SENECA LAKE, N.Y. . . s . « «6 + 


20 


21 





GEOLOGY 


lore Wael boa | 


THE MATERIALS OF THE EARTH AND PROCESSES 
WHICH AFFECT THEM 


CHAPTER I 
PRELIMINARY OUTLINE 


Geology 1s the history of the earth and its inhabitants. It treats 
of the rocks and of the agencies and processes which have made 
them, and from the rocks, their structures, and their fossils, it 
attempts to make out the stages through which the earth and the 
life which has dwelt upon it, have passed. 

Subdivisions. So broad a science has many subdivisions. 
Cosmic or Astronomic Geology treats of the outer relations of the 
earth; Geognosy treats of the materials of the earth, and its most 
important branch is Petrology, the science of rocks; Structural 
Geology deals with the arrangement of the rocks; Dynamic Geology 
deals with the forces involved in geologic processes; Physiographic 
Geology treats of the face of the earth, or topographic form; while 
Paleontologic Geology, or Paleontology, concerns itself with the fossils 
that have been preserved in the rocks, and with the faunas and floras 
that have lived in the past. The succession of events in the earth’s 
history constitutes Historical Geology, which is worked out chiefly 
from the succession of beds of rock formed through the ages, and 
from the fossils they contain. Besides these general subdivisions, 
there are special applications of geologic knowledge which give rise 
to other terms. Thus Economic Geology is concerned with the 
industrial applications of geologic knowledge, and Mining Geology, 
a sub-section of economic geology, deals with the application of 

I 


2 , PRELIMINARY OUTLINE 


geologic facts and principles to mining. Other similar subdivisions 
might be mentioned. 

Dominant processes. ‘Three sets of processes, still in operation 
on the surface of the earth, have made much of the record on which 
the science is based. ‘These processes have been designated dzas- 
trophism, vulcanism (volcanism), and gradation. Diastrophism 
includes all movements of the outer parts of the lithosphere, whether 
slow or rapid, gentle or violent, slight or extensive. Many parts 
of the land, especially along coasts, are known to be sinking slowly 
relative to the sea-level, while other parts are known to be rising. 
The fact that sediments originally deposited beneath the sea now 
exist in some places at great elevations, together with the fact that 
certain areas which were once land are now beneath the sea, proves 
that similar changes have taken place in the past. Earthquakes 
are another illustration of diastrophism. Vulcanism includes all 
processes concerned with the movements of lava and other volcanic 
products, whether they issue at the surface or not. Vulcanism and 
diastrophism may be closely associated, for many local movements 
are associated with volcanic eruptions. Gradation includes all 


those processes which tend to bring the surface of the lithosphere’ 


to a common level. Gradational processes belong to two categories 


— those which level down, degradation, and those which level up, 


aggradation. ‘The transportation of material from the land, whether 
by rain, rivers, glaciers, waves, or winds, is degradation, and the 
deposition of the sediment, whether on the land or in the sea, is 
aggradation. Degradation affects primarily the higher parts of 
the lithosphere, and aggradation the lower. | 


THE EARTH IN THE SOLAR SYSTEM 


Though supremely important to us, the earth is but one of the 
minor planets which revolve about the sun. Of the eight planets, 
four, Jupiter, Saturn, Uranus, and Neptune, are much larger than 
the earth, while three, Mercury, Venus, and Mars, are smaller. 
There are hundreds of asteroids, but all together they do not equal 
the mass of the smallest planet. Jupiter, the largest planet, has 
more than three hundred times the mass of the earth. The earth’s 
position is in no sense distinguished, for it is neither the outermost 
nor the innermost, nor even the middle planet. In the inner group 
of four to which it belongs, it is the largest. Its average distance 


- 


THE EARTH IN THE SOLAR SYSTEM 4 


from the sun is about 92.9 million miles, and its period of revolu- 
tion, 365% days, is longer than that of any other one of the inner 
planets, and shorter than that of any one of the outer group. The 
orbit of the earth, like the orbits of the other planets, is an ellipse. 
The inclination of the earth’s axis, nearly 231%°, is less than that of 
the axis of some planets, and more than that of others. 

The earth is peculiar in having one unusually large satellite, 
which has a mass !/8: of its own. The larger planets have several 
satellites whose combined mass exceeds that of the moon, and a few 
individual satellites may be larger than the moon; but no other is 1/81 
of the size of the planet about which it revolves. The moon has 
played an important part in the history of the earth, for it is the 
chief cause of tides, and tides are efficient in the wear of the shores 
of the oceans and in the distribution of marine sediments. Tides 
probably have been important ever since the ocean came into 
existence. 

The most important external relation of the earth is its depend- 
ence on the sun. Its mass is less than 1/300000 that of the sun, upon 
which it depends for nearly all its heat and light, and, through 
these, for nearly all of the activities that have determined its history. 
A little heat and light are received from other bodies, and an im- 
portant source of energy is found in the interior of the earth itself; 
yet all of these are so far subordinate to the great flood of energy 
which comes from the sun, that they are quite insignificant. The 
dependence of the earth on the sun has been intimate throughout 
its past history, and its future is locked up with the destiny of that 
great luminary. 

Meteorites. There are multitudes of small bodies, called mete- 
orites, passing through space in varying directions and with varying 
velocities. Great numbers of these reach the earth daily as “shoot- 
ing stars.” Some meteorites revolve about the sun like planets, but 
some of them do not belong to the sun’s family. Some consist 
almost wholly of metal, chiefly iron alloyed with a little nickel; 
some consist of metal and rock intimately mixed; and some consist 
wholly of rock. Since meteorites are thought to throw some light 
on the early history of the earth, they are of interest to the geologist. 
The amount of material added to the earth by the infall of meteorites 
is now slight compared with the whole body of the earth; but their 
contributions in the past may have been greater. 


4 PRELIMINARY OUTLINE 


THE GRAND DIVISIONS OF THE EARTH 


The constitution of the earth. The materials of the earth fali 
into three grand divisions: (1) The atmosphere, (2) the hydros phere 
(water sphere), and (3) the lithosphere (rock sphere). 

The atmosphere. Since the atmosphere is a part of the earth, 
its history falls within the province of geology. It is an intimate 
mixture of (1) all those substances that do not become liquid or 
solid under the temperatures and pressures which exist at the earth’s 
surface, together with (2) such transient vapors as the various 
liquid and solid substances of the earth throw off. The first are 
the principal gases of the atmosphere, and consist of nitrogen about 
78 parts, oxygen about 21 parts, carbon dioxide about .o3 part, 
together with small quantities of argon, and several other sub- 
stances. Chief among the second group is water vapor, which 
varies greatly in amount from time to time and from place to place. 
Here, too, belong the gases which issue from volcanoes, and many 
volatile organic substances. Dust and other matter suspended in 
the air are regarded as impurities rather than constituents of the 
atmosphere; but they are important because they affect the tem- 
perature and light of the air, and the condensation of its moisture. 

The mass of the atmosphere is estimated to be 1/1200000 of the 
total mass of the earth. It exerts a pressure of about fifteen pounds 
per square inch at sea-level. Its density decreases upward, but its 
actual height is not known. There is no direct evidence of its 
existence above a few hundred miles, but there are theoretical 
grounds for believing that it reaches much greater heights. 

Geologic activity. The atmosphere is the most mobile and active 
of the three great subdivisions of the earth. Its direct and indirect 
effects on water and rocks are so great that it must be regarded as 
one of the great agents of change in the earth’s history. The func- 
tion of the atmosphere in sustaining life and promoting all that 
depends on life is obvious. 

The hydrosphere. The water which lies upon the surface of 
the solid earth is about 1/4950 part of the earth’s mass. Were the 
solid part of the earth perfectly even, this amount of water would 
make a universal ocean a little less than two miles deep; but owing 
to the unevenness of the lithosphere, most of the water is gathered 
in the great basins which affect its surface. These basins are all 
connected, so that anything which changes the level of the water 
in one, changes it in all. 


GRAND DIVISIONS OF THE EARTH 5 


The area of the oceans is estimated at 143,259,300 square miles, 
or about 72% of the earth’s surface. The area of the true oceanic 
basins is only about 133,000,000 square miles, but the basins are 
somewhat more than full, and the ocean water overflows them, 
lapping up on the continental shelves to the extent of more than 
I0,000,000 square miles. If the uppermost 600 feet of the ocean 
water were removed, the true ocean basins would be just full. 
About 4/s of the ocean has a depth of more than a mile, and more 
than half of it a depth exceeding two miles. Its greatest depth is 
nearly six miles, and its average about two and one-half miles. 

The shallow waters which lie upon the continental shelves, or 
extend into the interiors of the continents, such as the Baltic Sea 
and Hudson Bay, are epicontinental seas, for they lie upon the low 
borders of the continental platforms. Those detached bodies of 
water which occupy deep depressions in the surface are to be re- 
garded as true abysmal seas. Such, for example are the Mediter- 
ranean and Caribbean seas and the Gulf of Mexico, whose bottoms 
are as low as many parts of the true ocean basin itself. Besides the 
oceans, the hydrosphere includes all the water of streams and lakes, 
together with that which is in the pores and fissures of the litho- 
sphere. The waters of the earth become a true hydrosphere only 
when the ground water is considered. All other waters of the earth 
are small in amount, compared with the ocean. 

Of all geological agents operating on the surface, water is the 
most obvious and apparently the greatest. Through rainfall, surface 
streams, underground waters, and waves, water is constantly 
modifying the surface of the lithosphere, most obviously by carry- 
ing sediment from the higher land and depositing it in the various 
basins. The hydrosphere is the great agency for the degradation 
of the land and the building up of the basin bottoms. The beds of 
sediment which it lays down follow one another in orderly succes- 
sion, each later one lying on an earlier. In this way, they form a 
time record. Relics (shells, bones, etc.) of the life of each age are 
embedded in the sediments, and record the history of life from age 
to age. The historical record of geology is dependent largely on 
the fact that the waters have buried, in systematic order, relics of 
the life of successive ages. 

The lithosphere. The atmosphere and hydrosphere are outer 
shells, rather than true spheres, though both penetrate the litho- 
sphere to some extent. The lithosphere, on the other hand, is an 


6 PRELIMINARY OUTLINE 


oblate spheroid with a polar diameter of 7,899.7 miles, and an 
equatorial diameter of about 26.8 miles more. Its equatorial cir- 
cumference is 24,902 miles, its meridional circumference 24,860 
miles, and its surface area about 196,940,700 square miles. Its 
average specific gravity is about 5.57. ‘The oblateness of the 
spheroid is the result of the rotation of the earth. 

The earth is not a perfect spheroid. Its equatorial diameters 
are not exactly equal, and the continental protuberances are, on the 
average, some three miles above the bottoms of the oceans. The 
forces or agencies which produced the continental platforms and 
abysmal basins, and the great undulations, foldings, and volcanic 
extrusions of both, are yet subjects of debate. 

It is customary to look upon the continents as the great features 
of the earth’s surface, but in reality the oceanic depressions are the 
master feature. They exceed the continental protrusions in breadth, 
and they are much farther below sea-level than the continents are 
above it. If the earth be regarded as a shrunken body, the settling 
of the ocean bottoms has doubtless been the greatest diastrophic 
movement. 

The following table shows the relative areas of the lithosphere 
above, below, and between certain levels. 


Per cent 
More than 6,000 feet above sea-level. ...........0.0+0+eevusees 203 
Between sea-level and 6,000 feet above.............+.0+s00e uae 2555 
Between sea-level and 6,000 feet below... ............-.0+acaee 14.8 
Between 6,000 and 12,000 feet below sea-level.................. 14.8 
Between 12,000 and 18,000 feet below sea-level...............-. 39.4 
Between 18,000 feet and 24,000 feet... ....6..1+) os cee ge 


From these estimates it appears that if the surface of the litho- 
sphere were graded to a common level by cutting away the conti- 
nental platforms and dumping the material in the ocean basins, 
bringing all to a common level, this level would be about 9,000 feet 
below sea-level. The continental platforms may be conceived as 
rising from this common plane rather than from the sea-level. 

The bottoms of the ocean basins have broad undulations ranging 
through many thousands of feet; but they have not those irregu- 
larities of form that give variety to land surfaces. The ocean bot- 
toms are also diversified by volcanic peaks, many of which consti- 
tute islands. From many of them, the solid surface slopes down 
rapidly to abysmal depths. Many of the volcanic islands are 


GRAND DIVISIONS OF THE EARTH 7 


isolated mountains whose heights and slopes would seem extraordi- 
nary, if the ocean were removed. 

The surface of the land is diversified similarly by broad undula- 
tions and volcanic peaks, as well as by narrower wrinklings and 
foldings of the crust, and ali of these irregularities have been carved 
into varied and picturesque forms by erosion. In this respect, the 
land differs radically from the bed of the sea. 

The outer part of the lithosphere is often called the crust of the 
earth. The old notion that it was the solid portion overlying a 
liquid part beneath is now generally abandoned. The crust is 
merely the outer, cooler portion of the lithosphere. Its thickness 
is undefined, but a shell several miles thick, and perhaps a few score 
miles, is penerally meant when this term is used. 

Materia's of the lithosphere. Mantle rock. The great ei of 
the lithosphere probably is composed of solid rock, but the solid 
rock is very generally covered by a layer of loose material such as 
soil, clay, sand, gravel, and broken rock, known collectively as 
mantle rock. The mantle rock of many places consists of the 
decayed products of underlying rocks. The upper part of mantle 
rock constantly is being blown away by wind and washed away by 
water, while the lower part is being renewed constzntly by the 





Fig. 1. Soil and subsoil arising from the decay of limestone resting on the 
sneven surface of the rock beneath. Southeastern Missouri. (Buckley.) 


8 PRELIMINARY OUTLINE 


decay of the rock below. The mantle rock of some other areas, as 
the northern part of North America and the northwestern part of 
Europe, consists chiefly of an irregular sheet of commingled clay, 
sand, gravel, and bowlders (drift) deposited by great glaciers, com- 
parable to that which now covers Greenland. In still other places, 
especially along the flood plains of streams, the mantle rock consists 
of deposits made by rivers. Along the shores of lakes and seas, 
there are beach gravels and sands. The thickness of the mantle 
rock varies from almost nothing to hundreds of feet (Fig. 1). 

Solid rock. Mantle rock is absent in some places, and there 
the surface of solid rock appears. It is common on the slopes of 
steep-sided valleys and mountains, on the slopes of cliffs which face 
seas or lakes, and in the channels of swift streams, especially where 
there are falls or rapids. In all lands inhabited by civilized peoples 
there are numerous wells and other excavations ranging from a few 
to several hundred feet in depth, and occasional wells and mine- 
shafts go much deeper. In these, and even in many of the shallower 
excavations, solid rock is encountered, and in most regions excava- 
tions as much as a few hundred feet deep reach it. We infer, there- 
fore, that solid rock is nowhere far below the surface. 

Varieties of solid rock. If the mantle rock were stripped from 
the land, the solid part beneath would be found to be made up of 
many kinds of rocks, all of which may be grouped into three classes. 





Fig. 2. Stratified rock. Trenton Limestone, Fort Snelling, Minn. (Calvin.) 


CLASSES OF ROCKS 9 


By far the larger part of the land surface would be of stratified rock, 
and the remainder of rocks without distinct stratification. The 
latter are divided into two great groups, igneous rocks, and meta- 
mor phic rocks. 

The essential feature of stratified rock (Fig. 2) is its arrangement 
in layers. The layers may be distinct or indistinct, and thick or 
thin. In many cases thick layers are made up of many thinner 
ones. In composition, most stratified rock corresponds somewhat 
closely with sediments now being carried from land and deposited 
in the sea; that is, these rocks are made up of gravel, sand, or mud, 
the particles of which are cemented together. The bedded arrange- 
ment of stratified rocks and of recent sediments is the same, and the 








ee ay ae > 
ee ae i 


Fig. 3. Diagrammatic representation of the relations of igneous rock to 
stratified rock. The igneous rocks, represented in black, have been forced up from 
beneath. 


markings on the surfaces of the layers, such as ripple-marks, rill- 
marks, wave-marks, etc., are identical. Furthermore, many of the 
stratified rocks of the land, like the recent sediments of the sea, 
contain the shells and skeletons of animals, and some of them the 
impressions of plants. Many of the relics of life found in the strat- 
ified rocks belonged to animals or plants which lived in salt water. 
Because of their structure, their composition, their distinctive 
markings, and the remains of life which they contain, it is confi- 
dently inferred that many, if not most, of the stratified rocks which 
lie beneath the mantle rock of the land originally were laid down 
in beds beneath the sea, and that the familiar processes of the pres- 
ent time furnish the key to their origin. 

Igneous rocks may be defined as hardened lavas. They sustain 
various relations to stratified rocks, as illustrated by Fig. 3, in which 
some of the igneous rock is represented as lying beneath the stratified 
rock, some above it, and some interbedded with it, while some cuts 
across its layers. From these relations it is possible to tell some- 


10 PRELIMINARY OUTLINE 


thing of the order in which the rocks were formed. Where stratified 
rocks are broken through by lavas, it is clear that the stratified 
rocks were formed first, and the lavas intruded later. Lava sheets 
intruded between beds of stratified rock can be told from those 
which flowed out on the surface and were subsequently buried, for 





Fig. 4. Metamorphic rock. (Ells. Can. Geol. Surv.) 


in the former case the sedimentary rocks, both above and below 
the igneous rock, were affected by the heat of the lava, while in the 
latter case only those below were so affected. 

Most metamorphic rock has cleavage; that is, a tendency to 
break in one direction rather than in another. The cleavage of 
metamorphic rock may look much like stratification, but it is really 
very different. The tendency to break along certain planes is not 
due to the fact that the rock was deposited in layers originally, as 
in the case of stratified rock, but is the result of the changes which 
the rock has undergone since it was formed. ‘The structure shown 
in Fig. 4 is known as schistosity —a structure characteristic of 
much metamorphic rock. Metamorphic rock may be derived from 
both igneous and sedimentary rocks. 

More commonly than otherwise, metamorphic rocks lie beneath 


THE EARTH’S INTERIOR Te 


sedimentary beds, or come to the surface from beneath them, 
Many of them are broken through by igneous rocks. 

Concerning the great interior of the earth, little is known except 
by inference. From the weight of the earth,' it is inferred that its 
interior is much more dense than its surface. From its . 
behavior under the attraction of other bodies, it is believed to be at 
least as rigid as steel. Its interior cannot, therefore, be liquid, in 
the usual sense of that term. From volcanoes, and from the tem- 
peratures in deep borings, it is inferred that the interior is very hot. 


1 The specific gravity of its outer portion is about 2.7, less than half that of the 
earth as a whole (5.57). 


CHAPTER II 
THE GEOLOGIC WORK OF THE ATMOSPHERE 


Since the atmosphere is a part of the earth, its activities and 
its history are proper subjects of geologic study. As a part of 
geology, the study of the atmosphere is restricted, commonly, to 
its effects on the other parts of the earth. The origin and history 
of the atmosphere must, however, be considered, in any thorough- 
going history of the earth. 

In the history of the earth, the atmosphere has played a part 
comparable to that of water, though its record is less clear. Its 
direct work is partly (1) mechanical and partly (2) chemical. Its 
indirect effects are even more important, for it furnishes the condi- 
tions under which (3) the sun produces its temperature effects, and 
(4) evaporation and precipitation take place. The atmosphere, 
' too, furnishes the necessary conditions for plants and animals, and 
the important influences that spring from them. 


MECHANICAL WORK 


The mechanical work of the atmosphere is accomplished chiefly 
through its movements. A feeble breeze moves particles of dust, a 
wind of moderate velocity blows dry sand, and exceptionally strong 
winds move small pebbles. 

The principal movement of the wind is horizontal; but every 
obstacle against which it blows deflects some of the air, and some of 
it is deflected upward. Furthermore, there are exceptional winds, 
in which the vertical element predominates. Particles of dust are 
caught by these upward currents, and carried to great heights. 
This facilitates their transportation great distances. 

Dust.! Transportation of dust by the wind is nearly universal. 
No house, no room, and scarcely a drawer is so tightly closed but 
that dust enters, and the movements of dust in the open are much 

1 Udden, Jour. of Geol., Vol. II, pp. 318-331; also Pop. Sci. Mo., Sept., 796. 

12 


MECHANICAL WORK ~ 13 


more considerable. The dustiness of the atmosphere in dry regions 
during wind-storms is familiar proof of the efficiency of the wind as 
a carrier of dust. 

Under special circumstances, it is possible to determine roughly 
the distance and height to which dust is carried. In the great 
eruption of Krakatoa in 1883, large quantities of volcanic dust 
(pulverized lava) were shot up to great heights into the atmosphere. 
The coarser particles soon settled; but many of the finer ones, caught 
by the currents of the upper air, were carried around the earth in 
15 days, and some of it traveled round the earth repeatedly. Its 
presence in the air was known by the historic red sunsets which it 
caused.! 

Dust from volcanoes is shot into the atmosphere rather than 
picked up by it. Dust picked up by the wind is perhaps transported 
as widely, but, after settling, its point of origin is less readily deter- 
mined. It would perhaps be an exaggeration to say that every 





Fig. 5. Vertical face of loess near Huang-tu-Chai in northern Shan-si. The 
vertical faces are the result of erosion. (Willis, Carnegie Institution.) 


square mile of land has particles of dust blown from every other 
square mile of dry land; but such a statement probably would in- 
volve much less exaggeration than might at first be supposed. 


1A brief account of the influence of the dust on sunsets is found in Davis’s 
Elementary Meteorology, pp. 85 and 119. 


14 GEOLOGIC WORK OF ATMOSPHERE 


Extensive deposits of wind-blown dust are known. Consider- 
able beds of volcanic dust, locally as much as 30 feet thick, are 
known in various parts of Kansas and Nebraska, hundreds of miles 
from the nearest volcanic vents. In some parts of China there is 
an extensive earthy formation, the loess (Fig. 5), in some places 
reaching a thickness of hundreds of feet, much of which is believed 
to have been deposited by the wind. ‘The loess of other regions 
has been referred to the same origin, and much of it is quite certainly 
eolian. From the flood plains of such rivers as the Missouri, clouds 
of dust are swept up and out over the adjacent high lands at the 
present time, especially when the surface of the flood plain has 
become dry after floods. This dust is very like loess, if, indeed, it 
is not loess. 

The transportation of dust is important wherever strong winds 
blow over dry surfaces free or nearly free of vegetation, and ccm- 
posed of earthy or sandy matter. Its effects may be seen in such 
regions as the sage-brush plains of western North America. The 
roots of the sage-brush hold the soil immediately about them, but 
between the clumps of brush where there is little other vegetation, 
the wind has in many places blown away the soil to such an extent 
that the base of each shrub stands up several inches, or even a foot 
or two, above its surroundings. Some of the mounds in this posi- 
tion are due partly to the lodgment of dust about the bushes (Fig. 6). 





Fig. 6. Shows the effect of sage-brush or other similar vegetation in holding 
sand or earth, or in causing its lodgment, in dry regions. 


EOLIAN SAND 5 


Since dust is carried to a considerable extent in the upper air, its 
movements and its deposition are affected but little by obstacles 
on the surface of the land, and when it falls it is spread more or less 
uniformly over the surface. | 

Much of the dust transported by the wind is carried out over 
seas or lakes and falls into them, causing sedimentation over their 
bottoms. No determinations of the amount of dust blown into the 
sea have been made, but it is safe to say that, if such determination 
were possible, the result would be surprising. 

Sand. Winds do not commonly lift sand far above the surface 
of the land, and its movement is therefore interfered with seriously 
by surface obstacles. A 
shrub, a fence, a ETE SJL TRL DR co a ea 
or even a stone may occa- G 4) ( ) 
sion the lodgment of sand 
in quantity, though it has 





little effect on dust. If the 
obstacle which causes the 
lodgment of sand presents a 
surface which the wind can- 
not penetrate, such as a wall, 
sand is dropped abundantly 
both on its windward and 
leeward sides (Fig. 7); but 
if it be penetrable, like an 
open fence, the lodgment 
takes place chiefly to lee- 
ward. 


Fig. 7. Diagram to illustrate the effect 
of an obstacle on the transportation and 
deposition of sand. The direction of the 
wind is indicated by the upper arrow. The 
lower arrows represent the direction of eddies 
in the air, caused by the obstruction. If the 
surface in which the obstacle was set was 
originally flat (dotted line), the sand would 
tend to be piled up on either side at a little 
distance from it, but more to leeward. At 
the same time, a depression would be hol- 
lowed out near the obstacle itself on either 
side. (After Cornish.) 


In cultivated regions, cases are known where, in a. few 


weeks of dry weather, sand has drifted into lanes in the lee of 
hedges to the depth of two or three feet, making it difficult for 


vehicles to pass. 
Dunes. 


In contrast with eolian dust, much eolian sand is aggre- 
gated into mounds and ridges called dunes. 


Some dunes are 200 


or 300 feet high, but many more are no more than ro or 20 feet in 

height. The shape of dunes depends, among other things, on the 

extent and form of the area furnishing the sand, the strength and 

direction of the wind, and the shape of the obstacles which occasion 

the lodgment. The shapes of the cross-sections of dunes are influ- 

enced by the strength and constancy of the winds. With constant 
1 Geog. Jour., Apr., 1910, p. 379. 


16 GEOLOGIC WORK OF ATMOSPHERE 


winds and abundant drifting sand, dunes are steep on the lee side 
(bc, Fig. 8), where the angle of slope rarely exceeds 25°. Under the 
same conditions, the windward slope is relatively gentle (ad). 


d o 





Fig. 8. Section of a dune showing, by the dotted line, the steep leeward (bc) 
and gentler windward (ab) slope. By reversal of the wind, the cross-section may 
be altered to the form shown by the line adc. (Cornish.) 


If the winds are variable, so that the windward slope of one time 
becomes the leeward slope of another, and vice versa, this form is 
not preserved. By reversal of the wind, the section abc may be 
changed to adc. Where the winds erode (scour) more than they 
deposit, other profiles are developed. The erosion profiles may be 





Fig. 9. Dunes at Longport, coast of New Jersey, showing the irregular forms 
developed by winds which erode. 


very irregular if the dunes are partially covered with vegetation 
(Fig. 9). 

Topography of dune areas. From what has been said, it is clear 
that the topography of dune regions varies widely, but it is always 
distinctive. Where the dunes take the form of ridges (Fig. 1, Pl. I), 
the ridges may be of essentially uniform height and width for con- 


THE CONTOUR MAP 17 


siderable distances. If there are parallel ridges, they may be sep- 
arated by trough-like depressions. Where dunes assume the form 
of hillocks (Figs. 2 and 3, Pl. I) rather than ridges, the topography 
is even more distinctive. In some regions depressions (basins) are 
associated with the dune hillocks. In some places they are hardly 
less notable than the dunes themselves. 


THE TOPOGRAPHIC MAP 


Since dunes as well as other topographic features are conveniently represented 
on contour maps, and since such maps will be used frequently in the following pages, 
a general explanation of them is here introduced. 

“The features represented on the topographic map are of three distinct kinds: 
(1) inequalities of surface, called relief, as plains, plateaus, valleys, hills, and 


aN 
gin Ws 
eh \ U MASS 3 hs 
RHNUNS mR 


ni NN 


) Sat 





Fig. ro. Sketch and map of the same area, to illustrate the representation of 
topography by means of contour lines. (U.S. Geol. Surv.) 


18 GEOLOGIC WORK OF ATMOSPHERE 


mountains; (2) distribution of water, called drainage, as streams, lakes, and swamps; 
(3) the works of man, called culture, as roads, railroads, boundaries, villages, and 
cities. 

“Relief. All elevations are measured from mean sea-level. The heights of 
many points are accurately determined, and those which are most important 
are given on the map in figures. It is desirable, however, to give the elevation 
of all parts of the area mapped, to delineate the horizontal outline, or contour, 
of all slopes, and to indicate their grade or degree of steepness. This is done by 
lines connecting points of equal elevation above mean sea-level, the lines being 
drawn at regular vertical intervals. These lines are called contours, and the 
uniform vertical space between two contours is called the contour interval. 
On the maps of the United States Geological Survey the contours and elevations 
are printed in brown (see PI. I). 

‘“The manner in which contours express elevation, form, and grade is shown 
in the preceding sketch and corresponding contour map, Fig. to. 

“The sketch represents a river valley between two hills. In the foreground 
is the sea, with a bay which is partly closed by a hooked sand-bar. On each side 
of the valley is a terrace. From the terrace on the right a hill rises gradually, while 
from that on the left the ground ascends steeply in a precipice. Contrasted with 
this precipice is the gentle descent of the slope at the left. In the map each of 
these features is indicated, directly beneath its position in the sketch, by contours. 
The following explanation may make clearer the manner in which contours delineate 
elevation, form, and grade: 

“y, A contour indicates approximately a certain height above sea-level. 
In this illustration the contour interval is 50 feet; therefore the contours are 
drawn at 50, 100, 150, 200 feet, and so on, above sea-level. Along the contour 
at 250 feet lie all points of the surface 250 feet above sea; along the contour at 
200 feet, all points that are 200 feet above sea; and so on. In the space between 
any two contours are found elevations above the lower and below the higher con- 
tour. Thus the contour at 150 feet falls just below the edge of the terrace, while 
that at 200 feet lies above the terrace; therefore all points on the terrace are shown 
to be more than 150 but less than 200 feet above sea. The summit of the higher 
hill is stated to be 670 feet above sea; accordingly the contour at 650 feet sur- 
rounds it. In this illustration nearly all the contours are numbered. ‘Where 
this is not possible, certain contours — say every fifth one — are accentuated 
and numbered; the heights of others may then be ascertained by counting up or 
down from a numbered contour. 

“9. Contours define the forms of slopes. Since contours are continuous 
horizontal lines conforming to the surface of the ground, they wind smoothly 
about smooth surfaces, recede into all re-entrant angles of ravines, and project 
in passing about prominences. The relations of contour curves and angles to 
forms of the landscape can be traced in the map and sketch. 

“3. Contours show the approximate grade of any slope. The vertical space 
between two contours is the same, whether they lie along a cliff or on a gentle 
slope; but to rise a given height on a gentle slope one must go farther along the 
surface than on a steep slope, and therefore contours are far apart on gentle slopes 
and near together on steep ones. 

“For a flat or gently undulating country a small contour interval is used; 
for a steep or mountainous country a large interval is necessary The smallest’ 


DUNES 19 


interval used on the atlas sheets of the Geological Survey is 5 feet. This is used 
for regions like the Mississippi delta and the Dismal Swamp. In mapping great 
mountain masses, like those in Colorado, the interval may be 250 feet. For 
intermediate relief contour intervals of 10, 20, 25, 50, and roo feet are used. 

“Drainage. Watercourses are indicated by blue lines. If the streams flow 
the year round the line is drawn unbroken, but if the channel is dry a part of the 
year the line is broken or dotted. Where a stream sinks and reappears at the 
surface, the supposed underground course is shown by a broken blue line. Lakes, 
marshes, and other bodies of water are also shown in blue, by appropriate con- 
ventional signs. 

“Culture. The works of man, such as roads, railroads, and towns, together 
with boundaries of townships, counties, and states, and artificial details, are 
printed i in.black.” From folio preface, U. S. Geol. Surv. 

Explanation of Plate I. In Fig. 1, Plate I (Five Mile Beach, 8 miles north- 
east of Cape May, N. J.), the contour eieeval is 10 feet. There is here but one 
contour line (the 1ro-foot contour), though this appears in several places. Since 
this line connects places 10 feet above sea-level, all places between it and the sea 
(or marsh) are less than 10 feet above the water, while all places within the lines 
have an elevation of more than 10 feet. None of them reaches an elevation of 
20 feet, since a 20-foot contour does not appear. It will be seen that some of the 
elevations in Fig. 1 are elongate, while others have the forms of mounds. (From 
Cape May, N. J., Sheet, U. S. Geol. Surv.) 

Fig. 2 shows dune topography along the Arkansas River in Kansas (Larned 
Sheet), and Fig. 3, dune topography in Nebraska (Camp Clarke Sheet), not in 
immediate association with a valley or shore. In Fig. 2 the contour interval is 20 
feet. All the small hillocks southeast of the river are dunes. Some of them are 
represented by one contour, and some by two. In Fig. 3, where the contour 
interval is also 20 feet, there are, besides the numerous hillocks, several depressions 
(basins). These are represented by hachures inside the contour lines. In some 
cases there are intermittent lakes (blue) in the depressions. There are two de- 
pression contours (4280 and 4260) within the contour of 4300, near Spring Lake. 
The bottom of the depression is therefore lower than 4260, but not so low as 4240. 


Migration of dunes.‘ By the transfer of sand from its windward 
to its leeward side, a dune is moved from one place to another, 
though continuing to be made up, in large part, of the same sand. 
In their migration, dunes may invade fertile lands, causing so great 
loss that means are devised for stopping them. ‘The simplest meth- 
od is to help vegetation to get a foothold in the sand. The effect of 
the vegetation is to pin the sand down. 

Where dunes migrate into a timbered region, they bury and kill 
the trees (Fig. 11). On the coast of Prussia a tall pine forest, cover- 
ing hundreds of acres, was destroyed between 1804 and 1827. At 
some points in New Jersey orchards have been so far buried within 
the lifetime of their owners that only the tops of the highest trees 

} Beadnell, Sand Dunes of the Libyan Desert, Geog. Jour., XX XV, 379, 1910. 


20 GEOLOGIC WORK OF ATMOSPHERE 





Fig. 11. Lee side of a sand dune, Cape Henry, Va. The dune is advancing on 
a forest and burying the trees. (Hitchcock.) ; 


are exposed. Trees and other objects once buried may be discov- 
ered again by farther migration of the sand ! (Fig. 12). 





Fig. 12. A resurrected forest. After burying and killing the forest, the sand 
was blown away, exposing the dead trees. (Myers.) . 


1 Cowles. The Ecological Relations of the Vegetation of the Sand Dunes 
of Lake Michigan. Botanical Gazette, Vol. XXVII, 1899. An excellent study 
of the relations of sand dunes and vegetation. 


= ; _ Angle soa PLATE | 


a4 


Fig. <1.—Dunes on 
coast of New Jer- 


Q 
8 ; 
= ‘s © 
: ° 
ties 
& > WS: Ry. > sey. Scale, about 


1 mile per inch. 


Contour interval, 
Het a 10. feet. (Cape 
May Sheet, U. 
= ' ag Ses S. Geol. Surv.) 
@ 
x 
+ 





Fie. 2—Dunes a- 
long Arkansas 
River in Kansas. 
Scale, about 2 Garfie fighd | 2A 





miles per inch. {ia ae? | 

Contour interval, fpr : 

20 feet. (Larned p* 

Sheet, U.S.Geol. |. f 

Surv.) ; #! 
\\ -_ 





Fic. 3.—Dunes in plains of Nebraska. Scale, about 2 miles per inch. 
Contour interval, 20 feet. (Camp Clarke Sheet, U. 8S. Geol. Surv.) 


PLATE Il 





Streams disappearing in the sand, gravel, ete., at the 
base of mountains in an arid region. Scale, about 
4 miles per inch. Contour interval, 200 feet. 
(Paradise, Nev., Sheet, U. S. Geol. Surv.) 


DUNES eH 


Distribution of dunes. Dunes are likely to be formed wherever 
dry sand is exposed to the wind. They are especially characteristic 
of the dry sandy shores of lakes and seas, of sandy valleys, and of 
arid, sandy plains. Along coasts, dunes are developed extensively 
only where the prevailing winds are on shore. Thus about Lake 
Michigan, where the prevailing winds are from the west, dunes are 
abundant and large on the east shore, and few and small on the 
west. Along valleys, dunes are most numerous on the far side as the 
prevailing winds blow. The dunes may be in the valleys, but in quite 
as many cases the sand is blown up out of the valley, and the dunes 
are on the bluffs above. Dunes probably reach their greatest 
development in the Sahara, but they are conspicuous in other arid 
sandy tracts, as in some parts of western Kansas and Nebraska, 
and in parts of Wyoming. 

Eolian sand is not all piled up into dunes. It may be spread 
somewhat evenly over the surface where it lodges. Eolian sand is 
much more widespread than dunes are. | 

Wind-ripples. The surface of the dry sand over which the 
wind has blown for a few hours is likely to be marked with ripples 
(Fig. 13). While the ripples are, as a rule, but a fraction of an inch 
high, they throw light on the origin of the great dune ridges. If 





Fig. 13. A ripple-marked sand dune in a western valley. (U.S. Geol. Surv.) 


22 GEOLOGIC WORK OF ATMOSPHERE 


the ripples be watched closely as the wind blows, they are found to 
shift their position gradually. Sand is blown up the gentler wind- 
ward slope to the crest of the ridge, and falls down on the other side. 
Wear on the windward side may be about equal to deposition on 
the leeward, and the result is the orderly progression of the ripples 
in the direction in which the wind is blowing, just as in the case of 
dune ridges. 

Abrasion. While the effect of the wind on sandy and dusty 
surfaces is considerable, its effect on solid rock is slight, except 


ier pk MN tae 





Fig. 14. Wind erosion. Cave rocks near Sierra La Sal, in Dry Valley, Utah. 
(Cross, U. S. Geol. Surv.) 


where sand and dust are driven against it. Rock worn by wind- 
blown sand acquires a surface peculiar to the agent accomplishing 
the work. If the rock is made up of lamine of unequal hardness, 
the blown sand digs out the softer ones, leaving the harder ones to 
project as ridges. The sculpturing thus effected on projecting 
masses of rock is picturesque and striking in some cases (Figs. 14 
and 15), and is most common in arid regions. 

Effect of wind on plants. Another effect of strong winds is 
seen in the uprooting of trees. The uprooting disturbs the surface, 
making the loose earth more readily accessible to wind and water. 
Organisms of various sorts (certain types of seeds, germs, etc.), as 
well as dust and sand, are transported far by wind. 


~ 


CHEMICAL WORK 23 





Fig. 15. Wind erosion. Casa Colorado, Dry Valley, Utah, between Abajo 
and La Sal Mountains. La Plata (Jurassic) sandstone. (Cross, U.S. Geol. Surv.) 


Indirect effects. Other dynamic processes are called into being 
by the atmosphere. Winds generate both waves and currents, 
which are effective agents in geologic work. The results of their 
activities are discussed elsewhere. 


CHEMICAL WORK 


The chemical work of the atmosphere is accomplished prin- 
cipally in connection with water. Dry air has little chemical effect 
on rocks or soils. ‘The important chemical changes wrought by the 
atmosphere are oxidation, carbonation, and hydration. Oxidation, 
as used in this connection, is the union of oxygen with some con- 
stituent of the rock, forming an oxide. Carbonation is a union of 
carbon dioxide of the air with constituents of the rock, forming 
carbonates. Hydration, similarly, is the union of water with con- 
stituents of the rock. Oxidation and hydration may go on at the 
same time. Thus when iron rusts, oxygen and water both enter 
into combination with the iron. In most cases these chemical 
changes result in breaking up the rock, much as steel or iron is 


24 GEOLOGIC WORK OF ATMOSPHERE 


broken up when it rusts. A few other effects of the atmosphere 
may be noted. 

Precipitation from solution. The water in the soil is constantly 
evaporating. Such substances as it contains in solution are depos- 
ited where the water evaporates, and where evaporation is long 





Fig. 16. Erosional forms characteristic of dry regions where erosion by the 
wind is effective. Fissure Canyon, north slope of the La Sal Mountains, Utah. 
The rock is Permian. (Cross, U. S. Geol. Surv.) 


continued, without re-solution of the substances deposited, the 
surface becomes coated with an efflorescence of mineral matter. 
An illustration is found in the alkali plains of certain areas in the 
western part of the United States. Certain substances, deposited 
when the water which held them in solution is evaporated, coat the 
pebbles and stones of some arid plains. In some places gravel is 
thus cemented into conglomerate. . 
Conditions favorable. Conditions are not everywhere equally 
favorable for the chemical work of the atmosphere. Since high 


EFFECT OF CHANGING TEMPERATURES 25 


temperatures facilitate chemical action, rocks are more readily 
decomposed by the chemical action of the atmosphere in warm than 
in cold regions. Changes of temperature tend to disrupt rock, and 
thus increase the amount of rock-surface exposed to chemical 
change. ‘The elements of the atmosphere are much more active 
chemically in moist than in dry regions. 

Though the chemical changes effected by the air are slow, their 
importance in the course of the earth’s long history has been very 
great. The amount of rock which has been thus disintegrated 
probably far exceeds all that is now above the sea. 


THE ATMOSPHERE AS A CONDITIONING AGENCY 


Temperature effects. Changes of temperature tend to break 
up rocks. The heating of rock by day and its cooling by night 
produce some such change in it as is produced by the quick heat- 
ing and cooling of glass. When the surface of the rock is heated, it 
expands, and a strain is set up between the hotter and more expand- 
ed part at the surface, and the cooler and less expanded part below.! 
This strain is enough to make the surface of the rock shell off in 
many cases. Daily variations in temperature are much more 
important than yearly variations, because they are much more 
common and take place more suddenly. Variations which do not 
involve the freezing of water are more important in long periods of 
time than those which do, because they are so very much more 
common. The daily range of temperature is influenced especially 
by (1) latitude, (2) altitude, and (3) humidity. (2) If other things 
were equal, the greatest daily ranges of temperature would be in 
low latitudes. (2) High altitudes favor great daily ranges of tem- 
perature, so far as the rock surface 1s concerned, for though the rock 
becomes heated during the sunny day, the thinness and dryness of 
the atmosphere allow the heat to radiate rapidly at night. Here, 
too, the daily range of temperature is likely to bring the wedge- 
work of ice into play. . Since the south side of a mountain (in the 
northern hemisphere) is heated more than the north, it is subject 
to the greater daily range of temperature, and the rock on this side 
suffers the greater disruption. Similarly, rock surfaces on which 
the sun shines daily are subject to greater disruption than those 


1Tt is the change of temperature of the rock surface, not the change of temper- 
ature of the air above it, which is considered here. 


26 GEOLOGIC WORK OF ATMOSPHERE ‘ 


much shielded by clouds. (3) The daily range of temperature is 
also influenced by humidity, a rock surface bécoming hotter by 
day and cooler by night beneath a dry’ atmosphere than beneath 
a moist one. Aridity, therefore, favors the disruption of rock by 
changing temperatures. The color of rock; its texture, and its 
composition also influence its range of daily temperature by in- 
fluencing absorption and conduction. ‘The disrupting ‘effects of 
changes of temperature are slight or nil where solid rock is pro- 
tected by soil, clay, sand, gravel, snow, or other incoherent material. 

In view of these considerations, the breaking of rock by changes 
of temperature should be greatest on the bare slopes of isolated 
elevations of rock, where the atmosphere is dry. All these 
conditions are not often found in one. place, but the disrupting 





Fig. 17. A mountain top, illustrating a common condition of the rock in 
mountain peaks. Med . 128 ns Sg 


EFFECT OF CHANGING TEMPERATURES ry 


effects of changing temperatures are best seen where several of them 
are associated. . 

The importance of this method of rock-breaking is rarely appre- 
ciated except by those familiar. with high and dry regions. Moun- 
tain climbers know that most high peaks are covered with broken 
rock to such an extent as to make their ascent dangerous to the 
uninitiated. High serrate peaks, especially of crystalline rock, are, 
as a rule, literally crumbling to pieces (Fig. 17). The piles of talus 
which lie on the slopes and at the bases of steep mountains are in 
some cases hundreds of feet in height, and their materials are in 





Fig. 18. Serrate mountain peaks with abundant talus. Cascade Mts., Wash. 


large part the result of the process here under discussion. Masses 
of rock, scores and even hundreds of pounds in weight, are some- 
times detached in this way, and started downward, and small pieces are 
much more common. ‘The sharp peaks which mark the summits of 
most high mountain ranges (Fig. 18) are largely developed by the 
process here outlined. Even in low latitudes and moist climates 
the effects of temperature changes may be seen. For example, 
thin beds of limestone at the bottoms of quarries have been known 
to expand under the heat of the sun, so as to arch up and break. 


28 GEOLOGIC WORK OF ATMOSPHERE 


The disruption of rock by changes of temperature is one phase 
of weathering. It tends to the formation of a mantle of rock 
waste, which, were it not removed, would soon completely cover 
the solid rock beneath and protect it from further disruption by 
heating and cooling; but the loose material thus produced becomes 
an easy prey to running water, so that the work of the atmosphere 
prepares the way for that of other eroding agencies. 

A thermal blanket. The atmosphere is a thermal blanket to 
the rest of the earth. Without it the heat of the sun would reach 
the earth with far greater intensity than now, and it would be radi- 
ated back from the surface almost as rapidly as received. During 
the night the earth would be far colder than any part of the earth is 
now. In passing through the atmosphere, parts of the radiant 
energy of the sun are absorbed. Of the remainder which reaches 
the surface of the earth, a part is radiated back into the air by which 
it is absorbed and retained. ‘The air thus distributes and equalizes 
the temperature. The constituents of the atmosphere which are 
most efficient in this work are water vapor and carbon dioxide, and 
the climate of the earth is believed to have been greatly affected by 
the varying amounts of these constituents, as well as by variation 
in the total mass of the atmosphere. 

Evaporation and precipitation. Perhaps the most important 
work of the atmosphere as a geologic agent lies in its relation to 
the evaporation, circulation, and distribution of water. Atmos- 
pheric temperature is the primary factor governing evaporation, an 
important factor in the circulation of the vapor after it is formed, 
and controls its condensation and precipitation. 

Mechanical effects of rain. In falling, the rain washes the 
atmosphere, taking from it much of the dust which the winds 
have lifted from the surface of the dry land. Not only this, but 
in passing through the atmosphere, the water dissolves some of 
its gases, so that when the rain reaches the land, the water is no 
longer pure. The dissolved gases enable it to dissolve various 
mineral matters on which pure water has little effect. 

As it falls on the surface of the land, the rain produces various 
effects of a mechanical nature. (1) It leaves on the surface the solid 
matter taken from the air. (2) Clayey soils, baked under the influ- 
ence of the sun, are softened by the rain, and more easily eroded by 
running water. (3) Under the influence of the expansion and con- 
‘traction caused by wetting and drying, the soils and earths on 


EFFECTS OF ELECTRICITY 29 


slopes creep slowly downward. (4) When rain falls on dry sand or 
dust the cohesion is at once increased, and shifting by the wind is 
temporarily stopped. 

Effects of electricity. Another dynamic effect conditioned by 
the atmosphere is that produced by lightning. In the aggregate, 
this result is unimportant; yet instances are known where large 
bodies of rock have been fractured by a stroke of lightning, and 
masses many tons in weight have sometimes been moved appreciable 
distances. Incipient fusion in very limited spots is also known to 
have been induced by lightning. Thus where it strikes sand it 
may fuse the sand for a short distance, and, on cooling, the partially 
fused material is consolidated, forming a little tube or irregular 
rod (a fulgurite) of partially glassy matter. Fulgurites are usually 
but a few inches long, and more commonly than otherwise a fraction 
of an inch in diameter. 


SUMMARY 


On the whole, the tendency of the work of the atmosphere, and 
of the work which is controlled by it, is to degrade the land, and to 
loosen materials of the surface so that they may be moved readily 
to lower levels by other agencies. The most important phase of 
the degradational work of the atmosphere is weathering, or the prep- 
aration of material for removal by other and more powerful agents 
of degradation. As we shall see, however, the atmosphere is not 
the only agent concerned in weathering. 

The wind has doubtless been an important agent in the trans- 
portation of dust and sand, wherever and whenever there was dry 
land, ever since an atmosphere has existed. If it has been as 
effective as now through all the untold millions of years since there 
have been land and atmosphere, the total amount of work which 
it must have done is past calculation. Wind-deposited sand, now 
cemented into solid rock, has been identified, even in very ancient 
formations. 


Laboratory work. The study of topographic and geologic maps, photographs, 
etc., illustrating wind work should be taken up in connection with this chapter. 
Plates XVI to XXII of Professional Paper 60 of the U. S. Geological Survey afford 
good illustrations of wind work. See also Interpretation of Topographic Maps, 
Exercise III, a laboratory manual (Henry Holt & Co.) which may be used with 
this text. 


CHAPTER III 
THE WORK OF GROUND (UNDERGROUND) WATER 


The average amount of precipitation on the land is estimated 
at about 40 inches per year. A part of this water sinks beneath 
the surface, a part forms pools or lakes, a part runs off at once, and 
a part of it is evaporated. The proportion of the rainfall which 
follows each of these courses depends on several conditions, among 
which are (1) the topography of the surface, (2) the rate of rainfall 
(or the rate at which snow melts), (3) the porosity of the soil or 
rock, (4) the amount of water which the soil contains when the rain 
falls or the snow melts, (5) the amount of vegetation on the surface, 
and (6) the dryness of the atmosphere. The steeper the slopes, 
the more rapid the rainfall, the less porous the soil, the wetter it is, 
and the less the vegetation, the more water will run off without 
sinking beneath the surface. 

The water which sinks into the ground becomes ground-water. 
The thousands of wells in lands peopled by civilized man, and the 
many springs which issue from the slopes of mountains and valleys 
prove that it is abundant and widely distributed. 

That ground-water is connected intimately with rainfall is 
shown by the following facts: (1) The level of water in wells com- 
monly sinks during droughts, and rises after rains; and the sinking 
is greater when the drought is long, and the rise greater when the 
rainfall is heavy. (2) Many springs discharge less water in times 
of drought, and others cease to flow altogether. (3) Rain-water is 
seen to sink beneath the surface, wherever the soil is porous. Sink- 
ing through the soil to the solid rock, it finds cracks and pores, and 
through them it descends to greater depths. Nowhere are the rocks 
which we see so compact and so free from cracks, when any con- 
siderable area is considered, as to prevent the sinking of water 
through them. 

The amount of ground-water in a given region does not depend 
entirely on the local rainfall. Ground-water is constantly moving, 
and some of it flows far from the place where it entered. Thus 


30 


GROUND-WATER SURFACE 31 


beneath the Great Plains of the West there is much water which 
fell on the eastern slopes of the Rocky Mountains. It has flowed 
beneath the surface to the plains, where some of it is drawn out for 
purposes of irrigation in regions where rainfall is deficient. 

Ground-water surface. Water-table. Ifa well 60 feet deep fills 
with water up to a point 20 feet below the surface, it is because the 
material in which it is sunk is full of water up to that level. When 
the well is made, the water leaks into it, filling it up to the level to 
which the rock (or subsoil) is itself full. This level, below which 
the rock and subsoil (down to unknown depths) are full of water, is 
known as the ground-water surface, or water-table. 

In a flat region of uniform structure and composition, the 
ground-water surface is essentially level, though it rises during wet 
weather, and sinks in times of drought. Its rise is due simply to 
the descent of rain water; but its sinking is due to several things: 
(1) Where there is growing vegetation, its roots draw up water from 
beneath; (2) evaporation goes on independently of vegetation; (3) 
the water is drawn out through wells, mines, etc., and runs out as 
springs; and (4) it flows underground from places where the water 
surface is higher to those where it is lower. In these and other 
minor ways the ground-water surface is depressed. 

A well sunk to such a level as to be supplied with abundant 
water in a wet season may dry up during a period of drought, be- 
cause the ground-water level is 
depressed below its bottom. 
Thus either well shown in Fig. 
19 will have water during a 
wet season when the water- 
level is at a; but well 1 will 
go dry when the water surface Fig. 19. Diagram illustrating the 
sinks to 0. fluctuation of the ground-water surfa-e; 

"Where the topography isnot Net weather ground-water level; b= 
flat, the ground-water surface is 
not level. As arule it is higher (though farther below the surface) 
under an elevation than under surrounding lowlands, as illustrated 
by Fig. 21. The reason is as follows: If a hill of sand is rained 
upon, most of the water falling on it sinksin. If the rain continues 
long enough the hill of sand will be filled with water, the water 
filling the spaces between the grains. The water in the hill tends 
to spread, but since the movement involves friction, the spreading 





32 GROUND-WATER 


is slow. With the spreading, the surface of the water in the sand 
sinks, and sinks fastest at the center where it is highest. If no 
water were added, the surface of the water in the hill would, in 





Fig. 20. Diagram showing how rain-water, falling in one place, may flow under- 
ground to another and there be brought to the surface. The layer a is porous, and 
water entering it in the mountains follows it to the plain. . 


time, sink nearly to the level of the water in the surrounding land; 
but at every stage preceding the last, the surface of the water would 
be higher beneath the summit of the hill than elsewhere, though 





Fig. 21. Diagram illustrating the position of the ground-water surface (the 
dotted line) in a region of undulatory topography. 


farther from the surface. In regions of even moderate precipitation 
the water-surface beneath the hills rarely sinks to the level of that 
in the lowlands about them, before it is raised by further rains. 

The water-surface beneath lowlands also sinks. Some of the 
water finds its way into valleys, some of it sinks to greater depths, 
and some of it evaporates; but since the water-surface beneath the 
elevations sinks more rapidly than that beneath the lowlands, the 
two approach a common level. ‘Their difference will be least at 
the end of a long drought, and greatest just after heavy rains. 

Depth to which ground-water sinks. The depth to which 
ground-water sinks has not been determined by observation. The 
deepest excavations are but little more than a mile deep, and at 
this depth the limit of water is not reached. There is a popular 
belief that water sinks until it reaches a temperature sufficient to 
convert it into steam; but except in places where hot lava lies near 
the surface, this belief does not appear to be well founded. Its 
descent probably is stopped in quite another way. 

Water descends through the pores and cracks of soil and rock, 


MOVEMENT OF GROUND-WATER 33 


and it doubtless goes down as far as they do. But it is probable 
that cracks do not go down more than a few miles, and that pores 
are limited to sirnilar depths. The reason for this is that rock, solid 
and unyielding as it seems, is yet mobile under sufficiently great 
pressure. If cracks or openings were formed in it at great depths, 
it is calculated that they could not persist, for the rock, under the 
pressure which exists there, would “flow” in and close them. The 
flow is, in effect, much like the flow of a stiff liquid. The outer zone 
of the earth, where cracks and cavities may persist, is the zone of 
fracture, and it is probable that the descent of water under ordinary 
conditions, is limited to this zone, variously estimated to have a 
depth of six to eleven miles.! 

Movement of ground-water.? Ground-water is in more or less 
continual movement. If all the water is pumped out of a well, it 
soon fills up again by inflow from the sides. Springs and flowing 
wells also demonstrate the movement of ground-water. Near 
the surface the movement is primarily downward if the rock through 
which it passes is equally permeable in all directions; but so soon 
as the descending water reaches the water-surface, its downward 
flow is checked, and its movement is partly lateral. 

Ground-water moves chiefly by slow percolation, for most of it 
is not organized into definite streams. Small streams are seen 
in some caves, and subterranean streams issue as springs in some 
places; but most streams which issue as springs probably have 
definite channels for short distances only, before they appear at the 
surface. The “reservoirs” from which artesian wells draw their 
supply are porous beds of rock, containing abundant water. As 
the supply is drawn off at one point, it is renewed by water entering 
elsewhere. Since the freedom of movement of ground-water is 
influenced greatly by the porosity of the rock, and since the rock is, 
on the average, most porous near the surface, the movement of 
ground-water is greatest near the surface, and less and less with 
increasing depth. Movement in the lower part of the subterranean 
hydrosphere doubtless is extremely slow. 

Amount of ground-water. The porosity of surface rocks varies 


1 Some recent experiments suggest that, at high temperatures and under great 
pressures, water may enter into combination with rock material, with contraction 
of volume. If so, water im combination (not free) may perhaps go below the zone 
of fracture. Barus. Bull. 92, U. S. Geol. Surv. 

2 For a full discussion of this subject see King, 19th Ann. Rept., U. S. Geol. 
Surv., Pt. II, and Slichter, Water Supply and Irrigation Paper 67, U. S. Geol. Surv. 


34 GROUND-WATER 


widely, and the porosity of but few has been determined.!. From 
such determinations as have been made, it is estimated that the 
average porosity of the outer part of the lithosphere is somewhere 
between five and ten percent. If the porosity diminishes at a con- 
stant rate to a depth of six miles (where it becomes zero), the average 
porosity to this depth would be half the surface porosity. An 
average porosity of 214% would mean that the rock might contain 
enough water to form a layer nearly 800 feet deep, if brought out 
to the surface.’ 

It is probable that the porosity decreases in more than an 
arithmetic ratio, both because the deeper rocks are not so generally 
of porous kinds as those at the surface, and because of the pressure 
which tends to close openings. For this reason it may be that the 
figure given above is too large, even for the land. ‘The porosity 
beneath the sea is probably less than that beneath the land, so 
that for the earth, 800 feet is perhaps too high a figure, and is not 
to be regarded as a measurement. 

Fate of ground-water. Most ot the water which sinks into the 
earth reaches the surface again after a longer or shorter journey. 
Some of it is evaporated from the surface direcily, some is taken 
up by plants and passed by them into the atmosphere, some issues 
in the form of springs, some seeps out, some is drawn out through 
wells, and much of the remainder finds its way underground to the 
sea or to lakes, seeping out beneath them. A small portion of the 
descending water enters into combination with mineral matter. 
It does not necessarily follow, however, that the total supply 
of water is for this reason decreasing. Minerals once hydrated 
may be dehydrated, the water being set free. Furthermore, con- 
siderable quantities of water in the form of vapor issue from volca- 
noes, and some volcanic vents continue to steam long after volcanic 
action proper has ceased. It is probable that some, and perhaps 
much of the water issuing from these vents has never been at the 
surface before. The amount of water reaching the surface of the 
earth for the first time from volcanoes, may, so far as now known, 

1 Buckley, Building and Ornamental Stones, Bull. IV, Wis. Surv.; Merrill, 
Stones for Building and Decoration. 

2 Slichter estimates that the ground-water is sufficient in amount to cover the 
earth’s surface to a depth of 3,000 to 3,500 feet: Water Supply and Irrigation 
Paper No. 67, U.S. Geol. Surv. Earlier estimates gave still higher figures. Fuller, 
in a recent estimate, places the amount at about 1oo feet: Water Supply and 
Irrigation Paper 160, U. S. Geol. Surv. 


WORK OF GROUND-WATER 35 


equal or even exceed the amount consumed in the hydration of 
minerals. | 


WORK OF GROUND-WATER 


Ground-water works chemically and mechanically, the chemical 
work being the more important. 

Chemical work. The chemical and chemico-physical action of 
ground-water may be grouped in several more or less distinct 
categories. 

1. The simplest result is the solution of mineral matter. Pure 
water dissolves little mineral matter; but the carbon dioxide ex- 
tracted from the atmosphere, and the products of organic decay 
extracted from the soil, give the water added power to dissolve. 
The solvent work of ground-water is shown by the fact that all 





Fig. 22. Sections of petrified logs, near Holbrook, Ariz. Age probably 
Jurassic. 


water from springs and wells contains mineral matter, while rain 
water is essentially free from it. The subtraction of soluble matter 
from rock tends to make it porous, and helps it to decay. 

2. One mineral substance in solution may be substituted for 


36 GROUND-WATER 


another extracted from the rock. Thus the lime carbonate of e 
shell imbedded in rock may be removed, molecule by molecule, 
and some other substance, such as silica, left in its place. Wher 
the process is complete, the substance of the shell has been com- 
pletely removed, though its form and structure are preserved in 
the new material. Buried logs may be converted into stone by the 
substitution of mineral matter for the vegetable tissue (Fig. 22). 

3. Materials dissolved from rock at one point may be de- 
posited in other rock elsewhere. ‘Thus a third type of change, 
addition, is effected. Rock may at one time and place be rendered 
porous by the subtraction of some of its substance, and the open- 
ings thus formed may later become the receptacles of deposits from 
solution. This is exemplified in the stalactitic deposits of many 
caves. Not uncommonly cracks and fissures are filled with mineral 
matter deposited by the waters which pass through them, making 
veins. 

4. A further series of changes is effected by ground-water 
when the mineral matter it contains enters into combination with 
the mineral matter through which it passes. In the long course of 
time, changes of this sort may be so great as to change rock com- 
pletely. 

Importance of solution. Calculations have been made which 
illustrate in a measure the quantitative importance of solution by 
ground-water. Most of the mineral matter dissolved in streams 
was contributed by ground-water (springs, etc.) flowing to them, 
and the amount in stream water is determined readily. The 
Thames River drains an area only about one-tenth as large as the 
State of New York, but it is estimated to carry about 1,500 tons of 
mineral matter in solution to the sea daily. From the uppermost 
20,000 square miles of its drainage basin, the Elbe is estimated to 
carry yearly about 1,370,000 tons of mineral matter in solution. 
Such figures make it clear that ground-water is an effective agent 
in the lowering of land surfaces. It is estimated that something 
like one-third as much matter is carried to the sea in solution as in 
sediment. 

The importance of the solution effected by ground-water is 
shown in another way. It is probable that most of the salt of the 
sea has been taken to it in solution by waters flowing from the land. 
The amount of salt is stupendous (Chapter VI). Furthermore, 
most of the limestone of the earth has been extracted from sea- 


PLATE Ill 


re 
3 
- f 
soe At Fe 
3 TER & 
\ s 
1 
mS 
: es = 
“T 
: . 
\ 
\ 
° i 
; 
to HEX 


— 


fe 


NQ 
8 ) ii Poe 
ae 


Shore of Lake Michigan just north of Chicago. Scale, 
about 1 mile per inch. Contour interval, 10 feet. (Highwood, II1., 
Sheet, U. 8..Geol. Surv.) 





PLATE IV 






A stream widening its valley by lateral planation. Scale, about 1 mile per 
inch. Contour interval, 20 feet. (Missouri, U. 8. Geol. Surv.) 


WORK OF GROUND-WATER 37 


water, whither the larger part of it was carried by streams, and the 
aggregate amount of limestone is far greater than the amount of 
salt in the sea. Some other sorts of rock, such as gypsum, of less 
importance quantitatively, have had a similar history. 

In general, solution is probably most effective at a relatively 
slight distance below the surface. In the mantle rock, the materials 
are as a rule less soluble than below, for in many places they 
represent the residuum after the soluble parts of the formation from 
which they originated were dissolved out. Below this zone, the 
rock contains more soluble matter, and the water, charged with 
organic matter in its descent through the soil, is in condition to 
dissolve it. At still greater depths the water has become saturated 
to some extent, and, so far forth, less active. At great depths, too, 
the movement is less free. Increased pressure on the other hand 
facilitates solution at great depths. 

Deposition of mineral matter from solution. Mineral matter is 
deposited from solution under various conditions. (1) Some of it is 
deposited by evaporation. ‘This is shown where water seeps out on 
arid lands. (2) Reduction of temperature may occasion deposition. In 
general, hot water is a better solvent of mineral matter than cold,! 
and if hot water issues with abundant mineral matter in solution, 
some of it is likely to be precipitated on cooling. (3) Certain 
plants cause the precipitation of mineral matter from solution, as 
about some hot springs in which alge grow in profusion. These 
little plants are a chief factor in the deposits about the hot springs 
of Yellowstone Park.? (4) A fourth factor involved in the deposi- 
tion of mineral matter is relief of pressure. Pressure increases the 
solvent power of water directly; it also increases the amount of gas 
which may be dissolved, and this in turn increases the solvent power 
of the water for some minerals. As water charged with gas comes 
to the surface, pressure is lessened, and some of the gas escapes. 
In numerous cases, mineral matter is then precipitated. (5) Pre- 
cipitation is sometimes effected by the mingling of waters contain- 
ing different mineral substances in solution. Such mingling of 
solutions is most common along lines of ready subterranean flow, 


1 This is not true in the case of minerals, such as the carbonates, dissolved and 
held in solution under the influence of gases dissolved in the water. 

2Weed. The Formation of Hot Springs Deposits; Excursion to the Rocky 
Mountains, and Ninth Ann. Rept. U.S. Geol. Surv., pp. 613-76; and B. M. Davis, 
Science, Vol. VI, pp. 145-57, 1897. : 


38 GROUND-WATER 


and while each portion of the water entering a crevice or porous 
bed might have been able to keep its own mineral matter in solu- 
tion, their mingling may involve chemical changes resulting in the 
formation of insoluble compounds, and therefore in deposition. 
This principle probably has been involved in the making of many 
veins of ore. 

The deposition of material held in solution is most notable at 
two zones, one below that of most active solution, and the other 
at the surface, where evaporation is greatest. Under proper con- 
ditions, however, deposition may take place at any level reached 
by water. 

Mechanical work. The mechanical work of ground-water is 
relatively unimportant. Where it flows in definite streams, the 
channels through which it flows are likely to be increased by me- 
chanical erosion as well as by solution. Either beneath the surface 
or after the streams issue, the mechanical sediment carried will 


be deposited. 


RESULTS OF THE WORK OF GROUND-WATER 


Weathering. Where the solvent work of ground-water is slight 
and equally distributed, its effect is to make the rock porous. 
If, for example, some of the cement of sandstone is dissolved, the 
rock becomes more porous; but if all the cement is removed, the 





Fig. 23. Diagram to illustrate the form and relations of caverns developed by 
solution. The black spaces represent caverns. Small limestone sinks are repre- 
sented at the surface where the roofs of caves have fallen in. 


rock is changed to sand. If a complex crystalline rock contains 
among its minerals some one which is more soluble than the others, 
that one may be dissolved. ‘This has the effect of breaking up the 


WORK OF GROUND-WATER 39 


rock, since each mineral acts as a binder for the rest. It may happen 
that no one of the minerals is dissolved completely, but that one or 
more of its constitutents is removed. Such change may cause the 
mineral to crumble, and so destroy the integrity of the rock. These 
are phases of weathering. 

Caverns.' In formations like limestone, which are relatively 
soluble, considerable quantities of material may be dissolved from 
a given place. Instead of making the rock porous, in the usual 
sense of the term, caverns are developed (Fig. 23). In their pro- 
duction, solution may be abetted by the mechanical action of the 
water passing through the openings which solution has developed. 

Caves are numerous in central Kentucky and southern Indiana, 
and the size of some of 
them, such as Mam- 
moth and Wyandotte, 
meevery “great. A 
ground-plan of Wyan- 
dotte (Ind.) Cave is 
shown in Fig. 24. The 
aeexceate length of its Fig. 24. Ground-plan of Wyandotte Cave. 
passageways 1S a numM- The unshaded areas represent the passageways. 
ber of miles. (21st Ann. Rept., Ind. Geol. Surv.) 

Deposition may take 
place in caves after they are formed (Fig. 25), or it may even go on 
at the same time that the cave is being excavated. Stalactites and 
stalagmites are common forms of cave deposits. A stalactite may 
start from a drop of water leaking through the roof of the cave. 
Evaporation, or the escape of gases in solution, results in the deposi- 
tion of some of the lime carbonate about the margin of the drop, in 
the form of a ring. Successive drops make successive deposits on 
the lower edge of the ring, which grows downward into a hollow 
tube through which descending water passes, making its chief de- 
posits at the end. Deposition in the tube ultimately may close it, 
while deposition on the outside, due to the water trickling down in 
that position enlarges it. 

Limestone sinks. Underground caves give rise to topographic 
features of local importance. If the roof of a cavern collapses, it 
causes a sink or depression in the surface. Some regions of lime- 
stone caves are affected by numerous sinks formed in this way. 

1 For a racy account of caverns see Shaler’s Aspects of the Earth. 





40 GROUND-WATER 


These limestone sinks (Figs. 26 and 23) as they are called, are con- 
spicuous in the cave region of Kentucky, and are well known in 
many other limestone districts. Some limestone sinks are made in 
other ways. 
Creep, slumps, and landslides. When the soil and subsoil on 
a slope become charged with water, they tend to move downward. 
When the movement is too slow to be sensible it is called creep; 
when rapid enough 
to be sensible, the 
material is said to 
slump or slide. 
This may happen 
when the slope on 
which water- 
charged mantle- 
rock lies is steep 
(Fig. 27). Some 
landslides have 
_ done great damage. 
Where a_ stream’s 
bank are high, and 
of unindurated ma- 
terial, such as clay, — 
considerable masses 
sometimes slump 
Fig. 25. Stalactites and stalagmites in Marengo from the. bank into 
Cave, southern Indiana. (Hains.) the river, or settle 
away slowly from 
their former positions. The same thing takes place on a larger 
scale on the slopes of steep mountains.! In creep and in landslides 
gravity is the force involved, and the ground-water only a condition 
which makes gravity effective. 





ORE-DEPOSITS 


Many ore-deposits are but a special result of the chemical 
work of ground-water, and are of interest because of their industrial — 
value. An ore is a rock that contains a metal that can be extracted. 
profitably, though the term is often extended to include unwork- 


» 1 Russell has emphasized this point in 20th Ann. U. S. Geol. Surv., Pt. Li: 
pp. 193-202, and Cross, 21st Ann. U. S. Geol. Surv., Pt. Il, pp. 129-150. 





Fig. 25. A sinkhole of recent development near Meade, Kan. (Johnson, U.S. 
- Geol. Surv.) 





_ Fig. 27. South face of Landslip Mountain, Colo. The protruding mass on 
the right has slumped down. (U.S. Geol. Surv.) 


42 GROUND-WATER 


able, lean bodies of ore material. The metal need not preponderate, 
or form any fixed percentage of the whole. Little gold ore contains 
more than a very small fraction of one per cent of the precious metal, 
while high-grade iron ore yields sixty-odd per cent of the metal. 
In iron ore, the metallic oxide or carbonate makes up nearly the 
whole rock; in gold ore, the metal is one of the least abundant 
constituents. 

Metals are disseminated widely through the rock substance of 
the earth, and even through the hydrosphere; but in their dis- 
seminated condition they are not ores. The concentration of the 
metals into workable richness, in accessible places, is the essential 
thing in the formation of ores. The degree of concentration re- 
quired is measured by the value of the metal. The chief points about 
ores to be considered in connection with ground-water are (1) the 
original distribution of the metallic materials, (2) their solution by 
circulating waters (or, rarely, by other means), (3) their transporta- 
tion in solution to the place of deposit, (4) their precipitation in 
concentrated form, and (5) perhaps their further concentration and 
purification by subsequent processes. Ores which originated in 
volcanic intrusions or from waters derived from lavas (magmatic 
waters) are mentioned but briefly here. 

Original distribution of ore material. For present purposes 
it is sufficient to regard all rocks concerned in ore-deposition as 
either igneous or sedimentary, and to inquire, first, how far ordinary 
igneous and sedimentary processes contribute to the segregation of 
ore material; and second, what the subsequent processes of local 
concentration are. 

Magmatic segregation. The segregation of metals in lava is 
known as magmatic segregation. In some instances masses of iron 
ore seem to have originated in this way. It is not improbable that 
the segregation of metallic iron and nickel, and perhaps other metals, 
may be common in the deeper parts of the earth, but it is not clear 
that many known ores originated in this way. It is probable, 
however, that there may be some segregation of metallic substances 
in lavas. While this segregation may not be rich enough to make 
ore, it may determine the places where subsequent concentration 
takes place, by the help of ground-water. 

Marine segregation and dispersion. In the formation of the 
sedimentary rocks there was notable metallic enrichment in some 
places. The ground-waters of the land, after their subterranean 


ORE-DEPOSITS 43 


circuits, carried to the seas various metallic substances in solution. 
In the main these substances appear to have been widely diffused, 
and to have been distributed very sparsely through the sediments, 
for sediments seem to contain less ore material than igneous rocks. 
There are however important exceptions to this general rule of 
sedimentary leanness. 

The iron-ore beds of Clinton age ranging from New York to 
Alabama, and appearing also in Wisconsin and Nova Scotia, form 
a stratum in the midst of ordinary sediments, and contain marine 
fossils. ‘The great iron-ore beds of Lake Superior also were sedi- 
mentary in origin, and so, probably, were most other important 
iron deposits. Not all sedimentary iron-ore deposits are of marine 
origin, and most of them are not clastic. Many of the sedimentary 
iron ores have been changed greatly from the condition in which 
the ferruginous matter was first deposited. In this change, ground- 
_ water has been the chief agent. Beds of clastic iron ore are known 
in Europe. The ore matter was in older rocks, and was segregated, 
mechanically, during sedimentation, because it was much heavier 
than other contemporaneous sediments. Its superior weight had 
much the same effect as greater coarseness. 

Some limestones appear to have been enriched locally, in a lean 
way, in lead and zinc, and rarely in copper, in the course of their 
formation. This lean enrichment at the time of deposition probably 
determined the development of ore regions later. The lead and zinc 
ore regions of the Mississippi basin have been regarded as areas of 
this sort, the subsequent concentration of the metal into ores being 
the work of ground-water. The lean enrichment accompanying 
sedimentation has been attributed to solutions of the metals brought 
to the sea from neighboring lands, the metals being then precipi- 
tated by organic action in the sea-water.'| This organic action may 
have been more effective in some areas than in others, because of 
the unequal distribution of life and the concentration of its decaying 
products.. 

Since it is reasonable to suppose that land-waters, on reaching 
the margins of the water-basins, must here and there find con- 
ditions favorable for the precipitation of their metallic contents, 
it is inferred that while the processes of sedimentation tended on 
the whole to leanness, they gave rise to (1) some very important 
ore-deposits, notably many iron ores, the greatest of all ores in 

1 Chamberlin. Geol. of Wis., Vol. IV, p. 5909, et seq., 1882. 


44 GROUND-WATER 


quantity and in industrial value, and (2) a lean enrichment of 
the sediments of certain other areas which, after subsequent 
processes of concentration of the metals by ground-water, became 
productive. | 

Origin of ore regions. From these considerations it appears 
that the fundamental explanation of many ‘‘mining regions” is 
to be found in (1) magmatic segregation, so far as the country rock 
is igneous, and (2) enrichment during sedimentation, so far as the 
rock is secondary. Either of these processes may, in rare cases, 
give rise to ores directly; but in most cases, further concentration 
of the metallic substances is necessary. This concentration is 
effected in various ways by the help of ground-water. 

1. Surface concentration. The simplest of all modes of con- 
centration takes place in the formation of mantle-rock. An in- 
soluble or slightly soluble metallic substance sparsely distributed 
through rock may be concentrated to working value by the decay 
and removal of the principal rock material, leaving the metallic 
matter in the residuary mantle. The tin ores of the Malay penin- 
sula! are examples. Crystals of tin oxide were originally scattered 
sparsely through granite and limestone. By the decay and partial 
removal of the rock, the crystals have accumulated in workable 
quantities. Certain gold fields and certain iron ores have acquired 
higher value in the same way; also certain ores of manganese, as 
those of Arkansas. Such residuary ores may be further concen- 
trated by running water, because the greater weight of the metals 
causes them to be left behind when the lighter substances are 
washed away, or because their greater weight causes them to be 
partially separated from the other sediments, in deposition. Gold 
placers are the best example. 

2. Purification. A different mode of concentration and puri- 
fication has affected some of the great iron deposits. As already 
stated, the iron compounds were originally parts of a sedimentary 
formation, and in beds. In some cases they were sufficiently pure, 
as first deposited, to be worked profitably; but in most cases they 
were affected by impurities. From such deposits the impurities 
have been dissolved by the percolation of waters, and at the same 
time, more of the valuable metal has been added. The great Bessemer 
iron-ore deposits of Lake Superior are examples. Originally impure 
silicates or carbonates, they have been converted into rich and 

1Penrose. Jour. of Geol., Vol. XI, pp. 135-155, 1903. 


ORE-DEPOSITS 4s 


phenomenally pure ferric oxides by ground-water. There are 
vast quantities of lean ores in the same region not thus purified and 
enriched.! 

3. Solution and re-precipitation. Ore material may be leached 
out of the surface-rock by water circulating slowly through it, and 
carried on until it reaches some substance which causes a reaction 
that precipitates the metallic matter. This substance may be a 
constituent of some rock which the circulating water encounters; 
but more commonly, the precipitation seems to be due to the 
mingling of waters charged with different mineral substances, the 
mingling inducing reactions which result in the precipitation of 
the ore. Precipitation does not necessarily follow such commin- 
gling; it takes place only when the mingling waters reduce the 
solubility of the ore material sufficiently. Changes of pressure and 
temperature also may enter into the process. 

Otherwise stated, the general process of underground ore forma- 
tion appears to be this: The permeating waters dissolve the ore 
material disseminated through the rock, and carry it thence into 
the main channels of circulation, usually the fissures, porous parts, 
or cavernous spaces. If precipitating conditions are found there, 
deposition takes place. The precipitating conditions may be 
merely changes of physical state, such as cooling or relief of pres- 
sure; but probably much more generally they are found in the 
commingling and mutual reaction of waters that have pursued 
different courses, and are differently mineralized. 

Location of greatest solution. Water circulation is probably 
very slight below the depth of a mile or two, and above that depth 
there is little reason for supposing that the rocks of one horizon are 
more metalliferous than others of their kind. Thus there is no 
assignable reason why the igneous or sedimentary rocks at the sur- 
face are not as rich in ore material as the igneous rocks two or three 
miles below. For a given amount of water, solvent action is prob- 
ably greatest where the temperature and pressure are highest, that 
is, in the deeper reaches of water circulation; but the amount of 
water passing in and out of the deeper zone is small compared with 
that of higher levels, and the total solvent action is quite certainly 
much greater in the upper zone than in the lower. At thesame time, 
the solutions in the upper zone are quite certainly more dilute than 
those below. The horizon of greatest solution doubtless lies be- 

1 Van Hise & Leith, various monographs of the U. S. Geol. Surv. 


46 GROUND-WATER 


tween the surface and a level slightly below the ground-water sur- 
face (p. 31); in other words, in the zone where atmosphere and 
hydrosphere co-operate. Surface-waters are charged with atmos- 
pheric and organic acids and other solvents, and their general 
effect upon the rocks is markedly solvent down to and somewhat 
below the permanent water-level. Concentration by residual 
accumulation may take place in this zone, as already noted, if the 
metallic compounds resist solution; otherwise this zone is depleted 
of its ore material by solution, and preparation is made for deposition 
elsewhere. 

Solution also continues to take place varyingly as the water 
descends below this zone of dominant solution, and extends prob- 
ably to the full depth of water circulation; but in the deeper circuit, 
precipitation also takes place, and with the waters taking up and 
throwing down material at the same time, it is difficult to estimate 
the balance of results. It is probable, however, that the result of ~ 
these processes is to promote the development of the higher ore 
values at levels near enough the surface to be accessible, and along 
the main lines of ground-water circulation. 

Influence of contacts. As many ore-deposits depend on a dis- 
solving state of the waters followed by a depositing state, it is 
obvious that conditions which favor changes of state and the com- 
mingling of different: kinds of water, are apt to be favorable to ore 
production. At any rate it is observed that many important ore- 
deposits occur at the contact of unlike formations, as for example 
at the contact of igneous rock with limestone. It is not to be in- 
ferred that such contacts are generally accompanied by workable 
ore-deposits, but merely that a notable proportion of workable 
ore-deposits occur at such junctions. It is rational to suppose 
that where the chemical nature of the two formations is in contrast, 
the waters that percolate through the one are likely to be mineralized 
very differently from those that course through the other, and that 
on mingling at the contact, reactions are liable to take place. When 
a valuable metallic substance is present, it may be involved and, 
by chance, suffer precipitation. Reactions are the more probable 
because the contact plane of formations is, in some cases, a plane 
of crustal movement, and hence more or less open and accom- 
panied by fractures, zones of crushed rock, and other conditions 
that facilitate circulation and offer suitable places for ore formation. 

The effect of igneous intrusions. A special case of much im- 


ORE-DEPOSITS 47 


portance arises where lavas are intruded into sediments that have 
previously been partially enriched in the ways described above. 
The igneous intrusion not only introduces new contact zones, and 
more or less fracturing, but it brings into play hot waters with 
their intensified solvent work, their more active circulation, and the 
reaction between waters of different temperatures. The special 
efficiency of these agencies is believed to be important in many 
cases. Furthermore the intruded lava may be rich in metallic 
substances, and so be a favorable site for later concentration. The 
magmatic waters themselves appear to be a source of important 
ore-deposits, as already noted, and the present tendency is 
to attach more and more importance to them. Ores deposited by 
magmatic waters are, in a sense, the product of magmatic segrega- 
tion (p. 42). 

The influence of rock walls. The rock walls themselves are 
thought to be a factor, in some cases, in the reactions which pre- 
cipitate ores. It appears that the effect of the wall may be to with- 
draw some constituent of the passing solution, and destroy its 
equilibrium in such a way as to cause the precipitation of metallic 
constituents. Once deposited on the walls, ore aids the further 
accretion of matter of the same sort. The effect of the rock wall 
here noted is sometimes called mass action. 

The special forms assumed by ores deposited from solution 
underground (veins, beds, etc.), are incidental to the local situation 
in which the precipitation takes place. 


SUMMARY 


All in all, ground-water is to be looked upon as a most important 
geological agent. When it is remembered that a very large part 
of all the water which falls on the surface of the earth, either in the 
form of rain or snow, sinks beneath the surface; that some of it 
sinks to a great depth; that much of it has a long underground 
course before it reappears at the surface; that it is everywhere 
and always active, either in subiracting from the rock through 
which it passes, in adding to it, in effecting the substitution of one 
mineral substance for another, or in bringing about new chemical 
combinations; and when it is remembered that these processes have 
been going on for untold millions of years, it will be seen that the 
total result accomplished must be great. The rock formations of 
the earth to the depths to which ground-water penetrates, are to 


Brg GROUND-WATER 


be looked upon as a sort of chemical laboratory through which 
waters are circulating in all directions, charged with many sorts of 
mineral substances. Some of the substances in solution are de- 
posited beneath the surface, and some are brought to the surface’ 
where the waters issue. Much of the material brought to the sur- 
face in solution is carried to the sea and utilized by marine organ- 
isms in the making of shells. Without the mineral matter brought 
to the sea by springs and river, many shell-bearing animals of 
great importance, geologically, would perish. Biologically, there- 
fore, as well as geologically, ground-water is of great importance. 
It is also of prime importance in the development of ores. 


SPRINGS AND ARTESIAN WELLS 


Springs. The term spring is applied to any water which issues 
from beneath the surface with volume enough to form a distinct 
current. If water issues so slowly as merely to keep the surface 
moist, it is seepage but not a spring. 

Many springs issue from the sides of valleys (Fig. 28), the bot- 
toms of which are below ground-water level. T hey are especially 
likely to issue at the surface of relatively impervious layers, and 
where the valley slopes cut joints, porous beds, or other structures 
which allow free flow of ground-water. 

Springs are classified in various ways, and the several classifi- 
cations suggest characteristics worthy of note. They are some- 
times classed as deep and shallow, but the idea involved in this 

















Fig. 28. Diagram showing conditions favorable for springs, in the side of a 
valley. P, porous rock, and I, impervious. Rain-water sinks to I, and, se 
along its surface, comes out as springs at S and S. 


grouping would be better expressed by strong and feeble. They 
are also classed as cold and thermal, the latter term meaning that 
the temperature is such as to make the springs seem warm or hot. 
The temperature of thermal springs ranges up to the boiling-point 
of water. Again, some springs are continuous in their flow, while 
others are intermittent. Most intermittent springs flow after periods 
of rain, but dry up during droughts. Springs are also classified as 


SPRINGS AND GEYSERS 49 


mineral and common. Mineral springs, in the popular sense of the 
term, are of two types: (1) Those which contain an unusual amount 
of mineral matter, and (2) those which contain some unusual min- 
eral. All springs which are not mineral are common. This classi- 
fication is not very significant, for all springs contain more or less 
mineral matter, and many springs which are ‘‘common”’ contain 
more mineral matter than some which are ‘‘mineral.”’ Mineral 
springs are themselves classified according to the kind and amount 
of mineral matter they contain. Thus saline springs contain salt; 
sulphur springs contain compounds (especially gaseous) of sulphur; 
calcareous springs contain abundant lime carbonate, etc. Medicinal 
springs are those which contain some substance which has, or is 
supposed to have, curative properties. 

Geysers. Geysers are intermittently eruptive hot springs. 
They occur only in volcanic regions (past or present), and in but 
few of them, being known only in the Yellowstone National Park, 
Iceland, and New Zealand. 

The cause of the eruption is steam. The surface-water sinks 
down until, at some unknown depth, it comes in contact with 
rock sufficiently hot to boil it. The source of the heat is not open 
to inspection, but it is believed to be the uncooled part of extruded 
or intruded lava. From what was said earlier in this chapter it 
is clear that geysers do not have their origin in water which sinks 
down to the zone of great heat, where the downward increment of 
heat is normal. 

The water of a geyser issues through a tube of unknown length. 
Whether the tube is open down to the source of the heat is not deter- 
minable, but water from such a source finds its way to the tube. 
Water may enter the tube from all sides and at various levels. 
The heating may precede or follow its entrance into the tube, or 
both. So far as the water is heated after it enters the tube, the 
point of most rapid heating may be at the bottom of the tube, or at 
some point above. If the water were converted into steam as fast 
as it enters the tube, steam would escape continuously, and there 
would be no geyser; but if the rock is only hot enough to bring the 
water in the tube to the boiling-point after some lapse of time, 
and after a good deal of water has accumulated, an eruption is 
possible. 

The exact sequence of events which leads to an eruption is not 
known, but a definite conception of the principles involved may 


50 GROUND-WATER 


be secured by a definite case. Suppose a geyser-tube full of water 
and heated at its lower end. As the water is heated below, con- 
vection tends to distribute the heat throughout the column of water 
above. If convection were free and the tube short, the result 
would be a boiling spring; but if the tube is long, and especially if 





Fig. 29. Giant Geyser, Yellowstone National Park. (Wineman.) 


convection is impeded, the water at some level below the surface 
may be brought to the boiling-point earlier than at the top. If 
even a little water in the lower part of the tube is converted into 
steam, the steam will raise the column of water above, and it will 
overflow. The overflow relieves the pressure on all parts of the 
column of water below the surface. If before the overflow there 
was any considerable volume of water essentially ready to boil, 
the relief of pressure following the overflow might allow it to be 
converted into steam suddenly, and the sudden conversion of a 
considerable quantity of water into steam would cause the eruption 
of all the water above it (Fig. 29). The height to which the water 


SPRINGS AND GEYSERS 51 


would be thrown depends upon the amount of steam, the size and 
straightness of the tube, etc. 

It is clear that everything which impedes convection in the 
geyser tube will hasten the 
period of eruption, since im- 
peded circulation will have the 
effect of holding the hot water 
down, and so of bringing the 
water at some level below the 
top more quickly to boiling. 
It follows that anything which 
chokes the tube, or which in- 
creases the viscosity of the 
water, hastens an eruption.! 

Some. geysers build up 
crater-like basins or cones 
(Figs. 30 to 32) about them- 
selves, the cone being of mate- 
rial deposited from solution Fig. 30. The cone of Lone Star Gey- 
(p. 37). The brilliant polos apne National Park. (U. S. 
of some of the deposits about 
the springs in the Yellowstone Park are attributed to the little 
plants which cause the deposition. When the water from any 
geyser or hot spring ceases to flow, the plants die and the colors 
disappear. 

The heating of geyser water must cool the lava or other source 
of heat below. As this takes place, the time between eruptions 
becomes longer and longer. In the course of time, therefore, the 
geyser must cease to be eruptive, and when this change is brought 
about, the geyser becomes a hot spring. Within historic time 
several geysers in the Yellowstone Park have ceased to erupt and 
new ones have been developed. There are something like 3,000 
vents of all sorts in this park, hot springs which are not eruptive 
greatly outnumbering geysers. 

A few geysers have somewhat definite periods of eruption. Of 
such ‘‘Old Faithful” is the type; but even this geyser, which 
formerly erupted at regular intervals of about an hour, is losing the 
reputation on which its name was based. Not only is its period 
of eruption lengthening, but it is becoming irregular, and the 


1Weed. Am. Jour. Sci., Vol. XX XVII, 1889, pp. 351-59. 





52 GROUND-WATER 


irregularity appears to be increasing. In the short time during 
which this geyser has been under observation its period has changed 
from.a regular one of 60 minutes, or a little less, to an irregular one 
of 60 to go minutes. In the case of some geysers, years elapse be- 
tween eruptions, and in some the date of the last eruption is so 
remote that it is uncertain whether the vent should be looked upon 
as a geyser or merely a hot spring. 





Fig. 31. Cone (or crater) of Grotto Geyser, Yellowstone Park. (Detroit 
Photo. Co.) ; 


Artesian wells. The terms artesian well and flowing well were 
synonymous originally; but any notably deep well is now called 
artesian. The artesian well which does not flow does not differ 
from a common well in principle, while the flowing well is really a 
gushing spring, the opening of which was made by man. 

Flowing wells ' depend upon certain relations of rock structure, 
water supply, and elevation. Generally speaking, a flowing well 
is possible in any place underlain by any considerable bed of porous 

1 Chamberlin. Geol. of Wis., Vol. I, pp. 689-97, and Fifth Ann. Rept., U. S. 


Geol. Surv., pp. 131-73. The former a brief, and the latter an elaborate, exposi- 
tion of the principles involved. 


ARTESIAN WELLS 53 


rock, if this rock outcrops at a sufficiently higher level in a region 
of adequate rainfall, and is covered by a layer or bed of relatively 
impervious rock. This statement involves four conditions, all of 
which are illustrated by Fig. 34, where a is the bed of porous rock. 
It is not necessary that the beds of rock form a basin, nor is it neces- 





Fig. 32. Deposit from a hot spring in Yellowstone Lake. (Fairbanks.) 





Fig. 33. Hot springs and geysers. Norris Geyser Basin, Yellowstone Park. 


34 GROUND-WATER 


sary, commonly, to take account of the character of the rock be- 
neath the porous bed which contains the water. 

The bed of porous rock is the ‘‘reservoir” of the flowing well. 
Sand or sandstone, and gravel or conglomerate, most commonly 





Fig. 34. Diagrams illustrating conditions favorable for artesian wells. In A, 
the porous bed a is in the form of a basin; in B, it merely dips. 


serve as the reservoirs. In order that they may contain abundant 
water they must have considerable thickness, and their outcropping 
edges must be so situated 
that water may enter free- 
ly, and be replenished by 
rain as the water flows out 
at the well. 

A relatively imprevious 
layer of rock above the 
reservoir (a, Fig. 34) is 
most important; otherwise 
the water in the reservoir 
will leak out, and there 
will be little or no ‘‘head”’ 
at the well site. Thus if 
the rock overlying stratum 
a were badly broken, the 
fractures extending up to 
the surface, the conditions 
would be unfavorable for 
flowing wells, for though 
wells might get abundant 


i wee ete : water, they would not be 
(U. & e. Se eee Drees Ole likely to flow. If the stra-. 





| tum next below the reser- 
voir is not impervious, some lower one probably is. No layer of 


ARTESIAN WELLS 55 


rock is more impervious than one which is full of water, and the 
substructure of any bed which might serve as a reservoir is usually 
full of water. 

If the outcrop of the reservoir is notably above the site of the 
well, and if it is kept full by frequent rains, the ‘‘head”’ will be 
strong, though the water at the well will not rise to the level of the 
outcrop of the reservoir. Experience has shown that an allowance 
of about one foot per mile of subterranean flow should be made. 
Thus if the site of a well is 100 miles from the outcrop of the water- 
bearing stratum, and 200 feet below it, the water will rise something 
like 100 feet above the surface at the well. This rule is, however, 
not applicable everywhere. The failure of the water to rise to the 
level of its head is due chiefly to the friction of flow through the rock. 
The more porous the rock the less the friction. The height of the 
flow is also influenced by the number of wells drawing on the same 
reservoir, on the degree of imperviousness of the confining bed above, 
etc. Flowing wells, many of them relatively shallow, are frequently 
obtained from unconsolidated drift. 

Map work. See Plates XC to XCIV of Professional Paper 60, U. S. Geological 


Survey, and Exercise IV, The Interpretation of Topographic Maps, a laboratory 
manual by Salisbury & Trowbridge. 


CHAPTER IV 
THE WORK OF RUNNING WATER 


Kivers are estimated to carry about 6,500 cubic miles of water 
to the ocean annually.! Since the average height of land is nearly 
half a mile, the waters which flow from it to the sea fall, on the 
average, nearly half a mile in their flow. Their total energy is 
therefore great, and they are.the great carriers of sediment from 
land to sea. The sediment which they carry is composed largely 





Fig. 36. Spokane River, 4 miles above Spokane, during flood. (Photo. by 
Tolman.) 


of decayed rock, but undecayed rock. is sometimes worn away, 
especially where streams are very swift. 

Though the flow of some streams is so gentle that they do not 
appear to work great changes in their valleys, others wear away 
their banks so rapidly that.the changes they produce may be seen 
from year to year, or, when the stream is in flood (Fig. 36), from 
day to day. Flooded streams occasionally sweep away dams, 
bridges, and buildings on their banks. The strong rods and beams 
of bridges and the steel rails of railways are bent almost as if they 
were twigs by the force of the occasional flood (Fig. 37). 

1 Murray, Scot. Geog. Mag. Vol. ITI. p. 70. 

56 


SOURCE OF RIVER WATER 57 


That the source of river water is the rain and snow which fall 
from the atmosphere may be inferred from various familiar phe- 
nomena. Thus (1) streams are more numerous in regions where 





Fig. 37. ‘Scene in the freight- ati of Kansas City after the flood of 1903. 
(U. S. Weather Bureau.) 
the rainfall is abundant than in those where it is scarce (Figs. 
38-39); (2) multitudes of small streams spring into being with 
each heavy fall of rain and with each period of rapidly melting 





ee $s" SEY GASESURNY 


Fig. 38 Fig. 39 
Fig. 38. Map showing the many streams of a humid region. Central Ken- 


tucky. The area is about 225 square miles. 
Fig. 39. Map showing the few streams of an arid region. Northern Arizona. 


The area is as great as that shown in Fig. 38. 


«8 WORK OF RUNNING WATER 


snow; (3) streams are notably swollen after rains, and most after 
heavy ones; and (4) many small streams which flow during wet 
weather dry up in times of drought, while others shrink. It is 
true that lakes, glaciers, and springs feed the rivers, but the lakes, 
glaciers, and springs derive their supply of water from precipitation. 
If the slope of a surface were perfectly even, the immediate 
run-off (the water which flows off without sinking beneath the 
surface) would flow in a sheet. There are slopes so smooth that 
water runs off them in this way; but on most slopes, even those 
which appear to be regular, there are small unevennesses, so that, 
although the run-off may start as a sheet, it is soon concentrated 
into rills and streamlets which follow the depressions. The smallest 
streamlets unite to form larger ones, and the little rills, after many 
unions with one another, reach valleys which have permanent streams. 
Streams which flow but part of the time, as after a rainstorm, dur- 
ing wet weather, or during but a part of the year, are temporary or 
intermittent streams. 
Every permanent 
stream and many 
temporary ones flow 
in depressions called 
valleys. Valleys are 
therefore about as 
numerous as streams. 
The very small de- 
pressions in which 
water runs. after 
showers only are 
called gullies if they 
are very small (Fig. 
40), or ravines if 
oe ; somewhat larger. 
ie Cue a gully developed by a single shower. ae Ane valle. 
and just as the tiny 

streamlets unite to form creeks and these to form rivers, so the little 
gullies, in which the smallest temporary streams flow, generally 
unite to form wider and deeper ones (Fig. 41). These, in turn, 
join one another and become ravines, which are but larger depres- 
sions of the same sort, and ravines lead to valleys just as gullies 





EROSIVE WORK 59 


lead to ravines. Valleys, like streams, usually end at the ocean or 
a lake; but in arid regions many of them end on dry land. 

There is, as a rule, some relation between the size of a valley and 
the stream which follows it, though this relation is not one which 
can be stated in mathematical terms. The large stream and the 





Fig. 41. Slope with numerous gullies, the smaller ones joining the larger ones. 


Scott’s Bluff, Neb. (U.S. Geol. Surv.) 


large valley go together so commonly, however, that the combina- 
tion cannot be accidental. 


EROSIVE WORK OF RUNNING WATER 


Wherever water flows over the land, it erodes the surface on 
which it flows, and the faster it flows, the greater its power of 
wear. ‘The rate of flow is determined chiefly by (1) the gradient 
(slope), (2) the amount and especially the depth of water, and (3) 
the amount of sediment (load) itis carrying. The steeper the gradi- 
ent, the deeper the water, and the less its load, the faster it flows. 
When it flows off in a sheet, as on a smooth surface, the depth of the 
water is slight, the flow not very swift (unless the slope is very steep), 
and the wear correspondingly slight. Such wear is sometimes 
called sheet erosion. 


60 WORK OF RUNNING WATER 


The Development of Valleys 


The growth of gullies. 1. If the slope of the surface is not uni- 
form the effect is very different. If there is, for example, a slight 
depression near the base of the slope (Fig. 42), more of the descend- 
ing water flows through it than over other parts of the surface. 
The greater volume of water in the depression gives it greater 
velocity; greater velocity causes greater erosion, and greater erosion 
deepens the depression. The immediate result is a gully or wash 
(Fig. 40). The gully, once started, tends to concentrate drainage 
in itself still more, and it is thereby enlarged. The water which 
enters it from the sides widens it; that which enters at its head 
lengthens it by causing its upper end to advance up the slope; and 
all which flows through it deepens it. The enlarged gully will 
gather more water to itself, and, as before, increased volume means 
increased velocity and increased erosion. As the gully grows, 
therefore, its increased size becomes the occasion of still further 
growth, and the gully is transformed into a ravine, which is no 
more than an enlarged gully. But growth does not stop with the 
ravine. Water from every shower gathers in it, and growth con- 
tinues until it becomes 
a valley. 

It was assumed in 
Mae a> the preceding para- 

Fig. 42. Diagram showing a slight meridional graph that the single 
Seba! in the surface of an otherwise even-sloped depression in the slope 
ew was meridional (Fig. 
42) and low on the slope; but almost any sort of depression in almost 
any position would bring about a similar result, since it would lead 
to concentration of the run-off. Had the original surface been 
marked by a single ridge instead of a depression, the effect on valley 
development would have been much the same, for a ridge, like a 
depression, would cause the concentration of the run-off along cer- 
tain lines, and therefore lead to the development of valleys. 

Under the conditions represented in Fig. 42 the lengthening of 
the drainage depression is effected chiefly at its upper end, the 
head of the valley working farther and farther back into the land. 
This method of lengthening is known as head erosion. But the 
lengthening of the valley is not always wholly by head erosion. 
The gully begins normally where concentration of run-off begins, 





DEVELOPMENT OF VALLEYS 61 


and if this is not at sea-level, the gully may be lengthening at both 
ends at the same time. This would have been the case, for exam- 
ple, had the depression of Fig. 42 been half-way up the slope. Val- 
leys developed under the control of surface slope are consequent 
valleys, and their streams are consequent streams. 

2. If the surface material of a slope is of unequal resistance, 
the water flowing over it will develop irregularities of slope, even 
if the slope was uniform at the outset. If the material of one 
part of a slope is less resistant than that elsewhere, the run-off 
will erode most there. The depression thus started will grow, and, 
as before, the gully may develop into a valley. In the presence 
of sufficient rainfall, therefore, either heterogeneity of slope or 
of material will cause the development of valleys. 

The permanent stream. It appears from the foregoing dis- 
cussion that a valley may be developed by the run-off of successive 
showers. If supplied from this source only, surface streams would 
cease to flow soon after the rain ceased to fall, and a valley might 
attain considerable size without possessing a permanent stream. 
The permanent stream is, as a rule, dependent on ground-water. 
When a valley has been deepened until its bottom is below the 
ground-water surface (p. 31), water seeps or flows into it from the 
sides. The valley is then no longer dependent on the run-off of 
showers for a stream. When the bottom of a valley is below the 
ground-water level of a wet season, without being below that of a 
dry one, it will have an intermittent stream. Many valleys are now 
in that stage of development where their streams are intermittent. 

As the valley of an intermittent stream becomes deeper, the 
periods when it is dry become shorter, and when it has been sunk 
below the ground-water level of droughts, it will have a permanent 
stream (3, Fig. 43). Since a valley normally develops headward, 
its lower and older portion is likely to have a permanent stream 
while its upper and younger part has only an intermittent one. So 
soon as a valley gets a permanent stream, the process of valley- 
enlargement goes on without the interruption to which it was sub- 
ject when the supply of water was intermittent. 

In general, a permanent stream at one point in a valley means 
a continuous stream from that point to the sea or lake to which 
the valley leads; but to this rule there are many exceptions, as where 
a stream heads in a region of abundant precipitation, and flows 
thence through an arid tract where the ground-water level is low 


62 WORK OF RUNNING WATER 


and evaporation great. In such cases, evaporation and absorption 
may dissipate the water gathered above, and the stream disap- 
pears (Pl. II). A stream like the St. Lawrence, which carries water 





Fig. 43. Diagram to illustrate the intermittency of streams due to fluctuations 
of the ground-water level. The water level aa would be depressed next the valley 
2-2, by the flow of water into the valley. The profile of the ground-water surface 
would therefore be aca and bdb rather than aa and 0b. 


from a great lake, does not depend on ground-water for its con- 
tinuous flow. Again, a stream which carries the water of a melting 
glacier may be permanent, even though not fed by springs. 

Other modes of valley development. Not all valleys are 
developed from gullies in the manner outlined above. 1. The out- 





Figs. 44 and 45. Diagram to illustrate one mode of valley lengthening. In 
Fig. 44 there are two small valleys, a and 6, and the former ends at the base of the 
steep slope. In Fig. 45, valley } is represented as having been lengthened so as to 
join a, and the two have become one. 


flow of a lake would develop a valley, and the valley might be in 
process of excavation all the way from the lake basin to the sea at 
the same time. A valley developed in this manner is not simply 
a gully grown big by head erosion, and the valley would not pre- 
cede the stream. 

If a narrow coastal plain is limited landward by a steeper slope, 
valieys might develop as shown in Figs. 44 and 45. Again, in 
some mountain regions valleys are formed by the up-folding of 


DEVELOPMENT OF VALLEYS 63 


parallel mountain ridges, leaving a depression between (Fig. 46). 
Drainage will appropriate such a valley, so that it becomes in some 
sense a river valley; 
but it is not a river 
valley in the sense in 
which the term has 
been used in the pre- 
ceding pages. It is 
rather a structural val- 
ley. A river valley 
may be developed 
in its bottom (a, Fig. 
46) and it may be in 
process of development throughout the whole length of the struc- 
tural valley at the same time. 

These illustrations do not exhaust the list of conditions under 
which valleys develop, but they suffice to show that valleys origi- 
nate and develop in different ways. 

Limits of growth. There are limits in depth, length, and width, 
beyond which a valley does not grow. A stream flowing to the sea 
tends to erode its valley to sea-level,’ but actually reaches the sea- 
level only near the coast. In length, the valley will grow as long 
as its head continues to work inland. If but a single valley affected 
a land area, the limit in length toward which it would tend would 
be the length of the land area in the direction of the valley’s axis. 
In general, valleys are limited in length by other valleys. The 
head of a valley works back until it reaches a point where erosion 
toward the valley in question is equal to erosion in the opposite 
direction. Here the divide 


becomes permanent (Fig. 47). i ee 
The width of a valley is in- so Se Op een 
creased chiefly by the side cut- Fig. AG racine , rare the 
. Owering Of a divide without shiiting it. 
ting of the stream, by the wash The crest of the divide is at a, b, and ¢ 
of the rain which falls on its successively. If the erosion was unequal 
slopes, and by the action of © the two sides, the divide would be 
AN ; shifted. 

gravity which tends to carry 

down to the bottom of the slope the material which is loosened above 
by any process whatsoever. The widening of valleys is limited 


Fig. 46. Structural valley with a river valley 
developing in its bottom. 


1 Great rivers, like the Mississippi, cut their channels somewhat below sea-level, 
for miles above their debouchures. 


64 WORK OF RUNNING WATER 


much as their lengthening is. Adjacent valleys grow wider until 
the tops of the intervening divides are reduced to lines. Then, 
if erosion is equal on the two sides, the divide is lowered without 
being shifted in position. 

The development of tributaries. Most considerable valleys 
have numerous tributaries. So soon as a gully is started, the 
water flowing into it from either side wears back the slopes. Any 
slight inequality of slope or material makes the erosion of the slopes 
unequal at different points, and unequal erosion in the slopes 
results in the development of tributary gullies. Some of these 
gullies develop into ravines and valleys, the same as their mains. 
Every new valley facilitates the run-off of the water which falls on 
the land, and so helps along erosion. 

Struggle for existence among valleys and streams. It is not 
to be inferred that every gully becomes a valley, nor that every 
small valley becomes 
a large one. The 
number of little gul- 
lies which develop on 
a slope may be very 
large (Fig. 41); but 
Fig. 48. Diagram illustrating how one gully takes the history of many 


others as a result of lateral erosion. The lines 1-4 of them is short. If 


represent, in cross-section, four stages in the develop- ‘ : 
ment of gullies a, b, and c. adjacent gullies are 


of unequal depth, the 
growth of the larger finally removes the divide between them, and 
they become one (Fig. 48). Again, a good map of the north shore 
of Lake Superior or the west shore of Lake Michigan shows a large 
number of small valleys and gullies (Pl. III). No equal stretch of 
coast has so great a number of large valleys. It therefore seems 
evident that of these many small valleys a few only will attain 
considerable size. 

Some young valleys work their heads back into the land faster 
than others, because of inequalities of slope and material. Ii 
valleys develop in ways other than by head erosion, the chances 
are also against their equal growth. If two streams, such as @ 
and c, Fig. 49, develop faster than the intermediate stream 8, it 
is clear that their tributaries may work back into the territory 
which at the outset drained into 6, so as to cut off the supply of 
water from the latter stream (compare a’b’c’, Fig. 50). As a result, 





DEVELOPMENT OF VALLEYS 65 


the growth of 6 will be checked, and ultimately stopped. Sim- 
ilarly other valleys, such as f (Fig. 49), will get the better of their 
neighbors, and many of the competitors, as 0’, d’, e’, and g’ (Fig. 
50), will soon drop out of the race. Between the stronger streams 





a 
@ 0 e 0 e pe bie 2 
Fig. 50 
eas es 
a 3 
Fig. 51 


Figs. 49, 50, and 51. Diagrams illustrating successive stages in the struggle for 
existence among streams. 


competition still goes on. If a’ and f’ (Fig. 50) develop faster than 
c’, its prospective drainage territory will be pre-empted by them 
(compare Figs. 50 and 51). Thus as the result of the unequal 
rate at which valleys are lengthened, the larger number of those 
which come into existence are arrested in their development. 
Piracy. Notall streams hold permanently the courses which they 


66 WORK OF RUNNING WATER 


establish for themselves in youth. Thus the Potomac River 
deepened its valley across the Blue Ridge (Fig. 52) faster than 
Beaverdam Creek deepened its valley. The head of the young 
Shenandoah River worked back and tapped Beaverdam Creek, 


A= 
KIT TATINNY 





Fig. 52 Fig. 53 
Figs. 52 and 53. ‘The capture of the head of Beaverdam Creek by the Shenan- 
doah River. Virginia-West Virginia. (After Willis.) 


diverting its head waters to the Potomac (Fig. 53). The Shenan- 
doah was a pirate, and Beaverdam Creek was beheaded. ‘The stream 
to which waters are diverted is increased in size, and the beheaded 
stream is correspondingly diminished. 


A Cycle of Erosion 


From what has preceded it is clear that the topography of a 
region undergoing erosion will change greatly from time to time. 
The first effect of erosion by running water is to roughen the sur- 
face by cutting out valleys, leaving ridges and hills. The final 
effect is to make it smooth again by cutting the ridges and hills 
down to the level of the valley bottoms. When this has been done 
the plain resulting is called a base-level. The time necessary to 
produce a base-level is a cycle of erosion. 

Base-level, peneplain, grade. The development of a base-level 
may be illustrated further in the light of the preceding discussion. 
Suppose a land surface affected by a series of parallel young valleys 
without tributaries (@ and b, Fig. 54). On either side of them 


CYCLE OF EROSION 67 


there are upland plains or plateaus. The profile of the surface 
about two adjacent valleys is represented in cross section by the 





wwe ELTA SSA LD a OO OS a ee Gyr SES, ee ea, —— Se = 


Fig. 54. Diagram to illustrate the leveling of the surface by valley erosion. 
The profile represented at the top shows two young valleys, 1 and 1, in an otherwise 
flat surface. In time these valleys will develop the cross-sections represented by 
2 and 2, and later those represented by 3 and 3, 4 and 4, etc. The divide between 
them may finally reach 5, and the surface is then nearly flat. 


uppermost line in Fig. 54. As the valleys a and 6 are widened to 
a’ and 0’, the adjacent uplands are narrowed correspondingly. 
When the valleys have attained the form represented by 3-3, the 
intervening upland has been narrowed to a ridge, and the valley 
flats have become wide. With continued erosion the ridge will be 
lowered still more, and in time the surface will approach a plain. 
In this condition it is known 

a peneplain. When the 
ridges are obliterated the 
peneplain passes into a base- 
leveled plain. 

Tributaries are almost 
sure to develop along each 
main valley and their heads 
work back across the up- Fig. 55. aes aoe tributaries in | 
lands between the main val- 2” ¢atly stage of development. 
leys, dissecting them into secondary ridges (Fig. 55). Tributaries 
develop on the tributaries, and these tertiary valleys dissect the 
secondary ridges into ridges of a lower order. This process of 
tributary development goes on until Sslomnesieh Ahi au the fourth, 
fifth, sixth, and higher orders BES a 
are ate (Fig. 56). Since 
the process of valley devel- 
opment under such circum- 
stances is also the process of 
ridge dissection, a stage is 
presently reached where the 
ridges are cut into such short. : ; ee 

Se on) qe Peteee tion of 


sections that they cease to a surface much dissected by the deveap- 
be ridges, and become hills ment of numerous tributaries. 













R= Te 
Fie ap 


68 WORK OF RUNNING WATER 


instead. Even then the processes of erosion do not stop, for rain- 
water falling on the hills washes the loose material from their 
surfaces, and starts it on its seaward journey. Thus the “‘ever- 
lasting hills’ are lowered, and, given time enough, will be carried 
to the sea. 

The base-leveled surface is not absolutely flat. The area 
reduced by each stream will have a slight slope down-stream, and 
from its sides toward its axis. The low divides between streams 
flowing in the same direction may, however, disappear altogether, 
for when valleys have reached their limits in depth, their streams 
ee F 5 do not cease to cut laterally. 

” Poe Meandering in their flat- 
peat + UES bottomed valleys, they may 

Soa > a reach and undercut their 
divides (P1.IV, and Fig. 57). 
By lateral planation, there- 
fore, the divides between 
streams may be entirely 
eaten away. 

' The terms ‘‘grade,” and 
) 3 \ “‘oraded plain,” and ‘‘base 
Fig. 57. Diagram showing streams in level” and ‘‘base-leveled 


adjacent valleys, undercutting the divide plain,’ are somewhat vari- 
between them. They may, in time, cut the 
divide away. ously, and therefore some- 


what confusingly, used. ‘‘A 
graded valley is one in which there is a condition of essential balance 
between corrasion and deposition.’”! Its angle of slope is variable 
and is dependent on the capacity of the stream for work, and on the 
work it has todo. A small river must have a higher gradient than a 
large one; a stream with much sediment must have a higher gradi- 
ent than one with little, and a stream with a load of coarse material 
must have a higher gradient than one with a load of fine. Thus 
the graded valley of the lower Mississippi has an inappreciable angle 
of slope; but the graded valleys of some of its small mountain 
tributaries have slopes of hundreds of feet per mile. Since both the 
size of the stream and the amount and coarseness of its load at a 
given place vary from time to time in the course of a cycle of erosion, 
it is clear that the inclination of the graded valley in a given place 
must vary from time to time. With the changing conditions of 

1Davis. Jour. of Geol., Vol. X, p. 87. 








Fic, 1 — Youthful topography in a region of slight relief. The valley of Maple 
Creek is narrow, and much of the area is unaffected by erosion. Seale, 
about 2 miles per inch, Contour interval, 20 ft. (N. D., U. S. Geol. Surv.) 


Irie AOE «Aa ane a) 
(et Springs, 


yf , BS “i Sprs 


nn 
Inspirgtion g 


Z L Point 
g® 


Veg Silver Cord 
x 
Hot Sprs Surf 


G 


VU; 
PP CG rece 





Fig. 2.—A young valley in a region of great relief. Scale, about 2 miles per 
inch, Contour interval, 100 feet. (U.S. Geol. Surv.) 


PLATE VI 





Fic. 1—A region in a mature stage of erosion. Scale, about 2 miles per 
inch. Contour interval, 100 feet. (Kentucky, U. 8S. Geol. Surv.) 


Co. 


EN tn ae 
; ~~ 
fs FRANCISCO 
Cc. 


Fic. 2.—A coast line developed chiefly by wave erosion. Seale, about 1 
mile per inch. Contour interval, 25 feet. (Tamalpais, Cal., Sheet, 
U.S. Geol. Surv.) 


t. Bonita 





CYCLE OF EROSION 69 
advancing years, the slope of a graded valley normally decreases. 
The same principles apply to graded surfaces outside of valleys. 

When a stream has brought the bottom of its valley to grade, 
it may be said to be at the level of base-level if the gradient is low; but 
a narrow valley flat at this level is not a base-level. This term, 
in the sense of a base-leveled plain, is applied to extensive areas 
only. Any extensive area degraded by running water to essential 
flatness is a base-level. Under later conditions of erosion, even with- 
out uplift, a base-leveled surface may be reduced (slightly) to a 
lower base-level. There is no sharp distinction between a base- 
level and an extensive graded surface of low gradient, if the latter 
was reduced by running water. 

The ocean may be looked upon as a barrier which in a general 





4 


Fig. 58. A shallow river valley in a plain. Cerro Gordo Co., Ia. Contrast 
with Fig. 59. (Calvin.) 

way limits the down-cutting of running water. Other barriers, 
such as lakes, and outcrops of hard rock in ‘a stream’s bed, 
have a comparable, though more local and temporary, effect on the 
development of valley plains above themselves. Plains thus de- 
veloped have been called temporary base-levels. 

Stages in a cycle of erosion. Since river valleys have a begin- 
ning and pass through various stages of development before the 
country they drain is base-leveled, it is convenient to recognize 
their various stages of advancement. Nor is this difficult. An 
old valley and a young one have different characteristics, and the 
one would no more be mistaken for the other by those who have 
learned to interpret them, than the face of an aged man would be 
mistaken for that of a child. 

Youth. ‘The cycle begins with the beginning of valley develop- 
ment, and at that stage drainage is in its infancy. The type of the 


70 WORK OF RUNNING WATER 

infant valley is the gully or ravine (Fig. 40). It has steep slopes and 
a narrow bottom. Plate III represents somewhat .older ravines, 
in contour. Asa valley is widened, lengthened, and deepened, 
it passes from infancy to youth. In this stage also the valleys are 
relatively narrow, and the divides between them broad. The valleys 
may be deep or shallow according to the height of the land in 





Fig. 59. Canyon of the Yellowstone below the falls. Yellowstone Park. 


which they are cut, and the fall of the water flowing through them; 
but in any case the streams flowing through them have done but 
a small part of the work they are to do before the country they 
drain is base-leveled. Figs. 58 and 59, respectively, represent 
youthful valleys in regions of slight and great relief. Fig. 1, Pl. V, 
shows youthful valleys in a region of slight relief, and Fig. 2, Pl. V, 


CYCLE OF EROSION a 


in a region of great relief. The uppermost line in Fig. 54 likewise 
represents topographic youth, as shown in cross-section. 

Not only are narrow valleys said to be young, but the territory 
affected by them is said to be in its topographic youth, since but a 
small part of the time necessary to reduce it to base-level has 
elapsed. An area is in its topographic youth when considerable 
portions of it are still unaffected by valleys. Thus the areas (as 
a whole), as well as the valleys, represented on Plate V, are in 
their topographic youth. It is often convenient to recognize 





Fig. 60. A valley much older than that shown in Fig. 59, Gray Copper Gulch, 
southwestern Colorado. (U.S. Geol. Surv.) 


various sub-stages, such as early youth, middle youth, and late 
youth, within the youthful stage of valleys and topographies. 

Youthful streams, as well as youthful topographies, have their 
distinctive characteristics. They are usually swift; their cutting 
is mainly at the bottom rather than at the sides, and their courses 
are often marked by rapids and falls. 

As valleys approach base-level, they develop flats. As 
valleys and their flats widen, and as their tributaries increase in 
number and size, a stage of erosion is presently reached in which 
but little of the original upland surface remains. The country 
is reduced largely to slopes, and in this condition the drainage and 
the topography which it has determined are said to be mature. 
Mature topography is shown in contours in Fig. 1, Pl. VI, where 
slopes rather than upland or valley flats, predominate. Mature 


72 WORK OF RUNNING WATER 


topography is also shown in Fig. 60, which illustrates the universal 
tendency of rivers in regions of notable relief to develop new flats 
well below the former surface of the region. 

The same processes which have made young valleys mature will 
in time work further changes. When the gradients of the valleys 
have become low and their bottoms wide, and when the intervening 
ridges and hills have become narrow and small, the drainage and 
the drainage topography have reached old age. This is illustrated 
by Fig. 1, Pl. VII, and in section by the third and lower lines in 
Fig. 54. Topographic old age may have a different expression; this 





Fig. 61. A peneplain near Camp Douglass, Wis. (Atwood.) 


is shown in Fig. 61, where most of the surface has been brought 
low. ‘The elevations which rise above the general plain are small 
in area, but have steep slopes. This expression of old-age topography 
is usually the result of unequal resistance of the rock degraded. 

The marks of old streams are as characteristic as those of young 
ones. They have low gradients and are sluggish. Instead of 
lowering their channels steadily, they cut them down in flood, and 
fill them up when their currents are not swollen. They meander 
widely in their flat-bottomed valleys (Pl. VII) and their erosion, 
except in time of flood, is largely lateral. 

The preceding discussion, and the illustrations which accom- 
pany it, give some idea of the topography which characterizes an 
area in various stages of its erosion history. Whether the valleys 
are deep or shallow in youth and maturity depends on the height of 
the land and its distance from the sea. The higher the land, and 


EROSION TOPOGRAPHY 73 


the nearer it is to the sea, the greater the relief developed by erosion. 
A plateau near the sea may become mountainous in the mature 
stage of its erosion history, while a plain in the same situation would 
only become hilly. A plateau in the heart of a continent would 
have less relief in maturity than one of equal elevation near the 
sea, since the grade-plain is higher in the former position than in 
the latter. 

Characteristics of river-shaped topographies. With the char- 
acteristics of river valleys clearly in mind, it is easy to say whether 
rivers have been the chief agents in the development of a given 
topography. River valleys are distinguished from other depressions 
on land surfaces by their linear form, and, leaving out of consider- 
ation the relatively insignificant inequalities in streams’ channels, 
by the fact that any point in the bottom is lower than any other 
point farther up stream in the same valley, and higher than any 
point farther down stream. The second point might be otherwise 
stated by saying that every valley excavated by erosion leads to’ 
a lower valley, to the sea, or to an inland basin. Streams which 
dry up, or otherwise disappear as they flow, constitute partial 
exceptions. If, therefore, the depressions on a land surface are 
linear, lead to other and deeper valleys, and finally to an inland 
basin or the sea, and if the elevations between these valleys are 
such as might have been left by the excavation of the valleys, it 
is clear that rain and rivers have been the chief factors in the 
development of the topography. If, on the other hand, a surface 
is characterized by topographic features which streams cannot 
develop, such as enclosed depressions, or hills and ridges whose 
arrangement is independent of drainage lines, other agents besides 
rain and surface streams have been concerned in its development. 

Note. For laboratory work see p. 120. 


ANALYSIS OF EROSION ! 


Erosion is the term applied to all processes by which earthy 
matter or rock is loosened or removed from one place to another. 
It consists of several sub-processes, namely, weathering, transporta- 
tion, corrasion, and corrosion. 

Weathering. Weathering is the term applied to nearly all those 

1 An excellent discussion of this subject is given by Gilbert in The Henry 


Mountains, pp. 99 et seq., and more briefly in the Am. Jour. Sci., Vol. XII, p. 85, 
et seq., 1876. 


74 WORK OF RUNNING WATER 


natural processes which tend to loosen or change the exposed sur- 
faces of rock. The inscriptions on exposed marble become fainter 
and fainter as time goes by, and finally disappear, because the 
rock in which the letters were cut has weathered away. In this 
case the weathering is effected partly by air and partly by water, 
two important agents of weathering. : 

The rain which falls upon the surface of exposed rock, and that 
which sinks through the soil to the solid rock below, dissolves 
slowly some of the constituents of the 
rock. This tends to make the rock 
crumble, much as mortar does when 
the lime carbonate which cements the 
sand is dissolved. The chemical 
changes effected by ground-water and 
the gases dissolved in it, also help to 
disintegrate the rock, as we have seen 
(p. 38). 

There are processes of weathering 
not due directly either to the atmos- 
phere or to water. Thus the roots of 
trees frequently grow in cracks of rocks 
(Fig. 62), and, increasing in size, act 
ciate siti like wedges. Water freezing in cracks 
Fig. 62. Tree growing in works in the same way. From the 
race Pstteda Tae meee "S faces of steep cliffs masses of rock are 

loosened frequently by the wedge-work 
of roots or ice, or by expansion and contraction due to changes of 
temperature. The quantities of debris at the bases of many cliffs, 
forming slopes of ¢alus (Fig. 63), testify not only to the importance 
of weathering, but also to the effectiveness of gravity in getting 
loosened material down. 

The importance of weathering in erosion is shown in many ways. 
Where the mantle rock is the product of the decay of the solid rock 
beneath, and this is the case over a large part of the earth’s surface, 
the soil and subsoil represent the excess of weathering over trans- 
portation. Since most of the earth’s surface is covered with soil 
and subsoil, it is clear that, on the whole, weathering keeps ahead of 
transportation. The loosening of rock by weathering greatly in- 
creases erosion, not only by running water, but by all other agents 
of erosion. ‘Though weathering is the first step in most erosion, it 





75 


ANALYSIS OF EROSION 





Talus slope, Utah. 


Fig. 63. 





Shows the downward creep of soil and slaty rock under the influence 


(U. S. Geol. Surv.) 


Fig. 64. 
of gravity. 


76 WORK OF RUNNING WATER 


is not the only one, and under some conditions erosion takes place 
without it. 

Transportation. A second element of erosion is transportation. 
The transportation of sediment is to be distinguished from the 
transportation of ma- 
terials in solution. In 
so far as mineral mat- 
ter is dissolved, it be- 
comes a part of the 
fluid of the stream. 
The quantity dissolved is too small to influence the mobility of the 
water sensibly. 

The sediment transported by a stream is either rolled along its 
bottom, or carried in suspension above the bottom. The coarser 
materials (gravel and sand) are carried chiefly in the former posi- 
tion, and the finer (silt and mud) largely in the latter. 

Transporting power and velocity. The transporting power of 
running water depends on its velocity. Swift streams have much 
greater power of transportation than sluggish ones, but transpor- 
tation does not always keep pace with transporting power. The 
Niagara at its rapids is a stream of great transporting power, but 
it carries little sediment, because there is little to be had. 

The velocity of a stream depends chiefly on three elements — 
its gradient, its volume, and its load. The higher the gradient, 
the greater the yolume, and the less the load, the greater the velo- 
city. The relation between gradient and velocity is evident; that 
between volume and velocity is illustrated by every stream in time 
of flood, when its flow is greatly accelerated. The relation between 
velocity and load is less obvious, but none the less definite. Every 
particle of sediment carried by a stream makes a draught on its 
energy, and energy expended in this way reduces the velocity. 
A muddy stream is never so swift as a clear stream of the same size 
would be, flowing in the same channel. 

How sediment is carried. Coarse materials, such as gravel- 
stones, are rolled along the bottom of the swift streams which carry 
them. ‘Their movement is by the impact of the water. The same 
is true to a large extent of sand grains. So far as concerns the 
material rolled along the bottom, it is to be noted that a stream’s 
transporting power is dependent on the velocity of the water at 
its bottom, which is much less than the velocity at the surface, and 
less than the average velocity. 





Fig. 65. Diagram of a valley, the top of which is 
ten times the width of the stream. 


TRANSPORTATION 77 


Particles of fine sediment, such as silt and mud, are carried by 
streams quite above their bottoms, as shown by the muddiness of 
many streams. Most particles of mud are small bits of mineral 
matter, the specific gravity of which is between two and three 


times that of water. 
come to rest at the bottom. 


Yet they do not sink through the water and 


A particle of sediment in running water is subject to two prin- 


cipal forces, that of the current 
which tends to move it nearly hori- 
zontally down stream, and that of 
gravity which tends to carry it to 
the bed of the stream. As a result, 
the particle tends to move in the 
direction which represents the re- 
sultant of these forces (Fig. 66). If 
a river were the simple straightfor- 
ward current which it is popularly 
thought to be, a particle in suspen- 
sion would reach its bottom in the 
time it would take to sink through 
an equal depth of still water; for the 





Ya 

Fig. 66. Diagram to illustrate 
the relative strength of the two 
forces acting on a particle in sus- 
pension. The arrows represented 
by full lines show the relative 
strength of the two forces when the 
stream’s velocity is about 5 miles 
per hour. No account is taken in 
the diagram of the viscosity of the 
water, or of the acceleration of 
velocity of fall. 


descent would be none the less cer- 
tain and scarcely less prompt because of the forward movement of 
the water. The current would simply be a factor in determining 
the position of the particle when it reached the bottom, not the 
time of reaching it. Very fine particles, like those of clay, sink less 
readily than coarser ones, because the former expose larger surfaces, 
relative to their mass, to the water through which they sink. But 
even such particles, unless of extraordinary fineness, would pres- 
ently reach the bottom if acted on only by a horizontal current and 
gravity. Since even sediment which is not of exceeding fineness is 
kept in suspension, it is clear that some other factor is involved. 
This is found, in part at least, in the subordinate upward currents 
in a stream. 

Where a bowlder occurs in the bed of a stream (Fig. 67) a part 
of the water which strikes it is forced up over it. If there are many 
bowlders, the process is repeated frequently, and the number of 
upward currents is great. Any roughness will serve the same pur- 
pose, and every stream’s bed is rough to a greater or less extent. 
Roughnesses at the sides of a channel start currents which flow 


78 WORK OF RUNNING WATER 


toward the center, and the varying velocities of the different parts 
of a stream serve a similar purpose. A river is therefore to be 
$e OOK Gee ee 
tude of currents, some 
rising from the bottom 
jie toward the top, some 


ee . 
VAnw. bm en Din descending from top to 
Fig. 67. Diagram to illustrate the effect of bottom, some diverging 
irregularities, a and b, in a stream’s bed, on the from the center to- 


current striking them. ward the sides and 
some converging from the sides toward the center. The sum of 
the upward currents is of course always less than the sum of the 
downward, so that the aggregate motion of the water is down slope. 

Sediment in suspension is held up chiefly by the upward currents, 
which, locally and temporarily, overcome the effect of gravity. 
The particles in suspension are constantly tending to fall, and fre- 
quently falling; but before they reach the bottom, many of them ar¢ 
carried up by subordinate currents, only to sink and be carried up 
again. Even if they reach the bottom, as they do frequently, they 
may be picked up again. It is probable that every particle of sedi- 
ment of such size that it would sink readily in still water is dropped 
and picked up many times in the course of any long river journey, 
and its periods of rest often exceed its periods of movement. 

Corrasion. ‘The mechanical wear effected by running water is 
corrasion. So long as the materials to be moved are incoherent, it 
is easy to understand how running water moves them. The water 
which flows over the surface of a cultivated field gathers earthy mat- 
ter, and the process is continued all the way to the channel of the 
stream. Thus sediment is gathered at the very sources of flow, 
and the stream gathers load from its bed wherever it flows with 
sufficient velocity over loose material. Streams also undercut 
their banks, and receive new load from the fall of the overhanging 
material. 

The larger part of the sediment of streams is made up of mate- 
rial loosened in advance by weathering; but many rivers wear rock 
which is not weathered, for the principal valleys of the earth are in 
solid rock, and many of them in rock of great hardness. How does 
the stream wear the solid rock? | 

When a stream flows over a rock bed, the wear which it accom- 
plishes depends chiefly on the character of the rock, the velocity 


— 


CORRASION 79 


of the stream, and the load it carries. If the rock is much divided 
by bedding planes and joint planes, the water of a clear stream of 
even moderate strength may dislodge bits of the rock. This con- 
dition of things is seen where streams run on beds of shale or slate. 
If the rock is hard and without bedding planes or joints, or if its 
layers are thick and its joints few, clear water is much less effective. 
If massive hard rock presents a smooth surface to a clear stream, the 
mechanical effect of even a swift current is slight. 

This general principle is illustrated by the Niagara River. Just 
above the falls the current is swift. When the river is essen- 
tially free from sediment, the surface of the limestone near the bank 
beneath it sometimes is distinctly green from the presence of the 
one-celled plants (fresh-water alge) which grow uponit. The whole 
force of the mighty torrent is not able to sweep them away. Were 
the stream supplied with a tithe of the sand which it is capable of 
carrying, it would not take many hours, and perhaps not many 
minutes, to remove the last trace of the vegetation. This illus- 
tration furnishes a clue to the method by which the erosion of solid 
rock in a stream’s bed is effected. 

The gravel rolled along the channel wears even solid rock, and 
as the moving stones wear the stream’s bed, they are themselves 
worn by impact both with the bed and with one another, and are 
reduced to rounded, water-worn forms. The particles broken off 
may make grains of sand, or, if very fine, particles of silt or mud. 
In the course of time the pebbles and cobbles rolled along may be 
literally worn out. 

The sediment carried in suspension, as well as that rolled along 
the bottom, wears the rock bed of a stream. ‘The coarser the sedi- 
ment and the stronger the current, the greater the wear. The 
gravel, sand, and mud carried by a stream are therefore the tools 
with which it works. Without them it is relatively impotent, so 
far as the abrasion of solid rock is concerned; with them, it may wear 
any rock over which it passes. 

Swift and slow streams corrade their valleys differently. The 
erosion of a swift stream is chiefly at the bottom of its channel. 
The sluggish stream lowers its channel less rapidly, or not at all, and 
lateral erosion is relatively more important. The result is that slow 
streams increase the width of their valleys more than the depth, 
while swift streams increase the depth more than the width. It 
follows that slow streams develop flats, while swift ones do not. 


80 WORK OF RUNNING WATER 


Not only is a slow stream more likely to have a flat, and therefore 
a better chance to meander, but it is more likely to take advantage 
of opportunities in this line, for a slow stream gets out of the way 
for such obstacles as it may encounter, while a swift stream is 
much more likely to get obstacles out of its way. 

Corrosion. In most cases the solution (and other chemical 
changes) effected by a stream is much less important than its me- 
chanical work. Only when conditions are unfavorable for the 
latter is solution the chief factor in the excavation of a valley. This 
may be the case where a stream’s bed is over soluble rock, such as 
limestone, and where the stream is clear, or its gradient so low that 
its current is sluggish. The solvent power of water is not influenced 
by the presence of sediment, though the presence of sediment offers 
the water a greater surface on which to work. 


CONDITIONS AFFECTING THE RATE OF EROSION 


With a given amount of water, the declivity, the character of 
the rock, and climate, are the principal factors influencing the rate 
of erosion. 

Declivity. In general, the greater the slope the more rapid the 
rate of erosion by running water, whether in the stream’s channel or 
on the slopes above. But high declivity does not favor every ele- 
ment of erosion. It favors some phases of weathering and hinders 
others, but it favors both transportation and corrasion. Both 
corrasive power and transportive power increase rapidly with in- 
crease of velocity, and under these circumstances, corrasion also 
will be increased if the water has tools to work with, and trans- 
portation will be increased if there is material which can be carried. 
Since high declivity greatly increases both the transporting and the 
corrading power of running water, and favors certain elements of 
weathering, it is clear that its aggregate effect is to favor erosion. 

Rock. The physical constitution, the chemical composition, 
and the structure of a rock formation influence the rate at which 
it is broken up and carried away. 

Physical constitution. ‘The constituents of clastic rocks may be 
firmly or weakly cemented. The less the coherence the more ready 
the disintegration, and the finer the particles the more easily they 
are carried away. If the materials carried are harder than the bed 
over which they pass, corrasion of the latter is favored. 

Chemical composition, Something also depends on the chemical 


RATE OF EROSION 81 


composition of the rock, since this affects its solubility and its rate 
of decomposition. The more soluble the rock, the larger the pro- 
portion of it which will be taken away in solution; but it does not 
follow that the most soluble rock will be most rapidly eroded, since 
the rate of erosion depends on abrasion as well as solution, and 
a rock which is readily soluble, as rocks go, may be less easily 
abraded than one which is made of discrete and insoluble par- 
ticles bound together by a soluble cement. In such rocks, for 
example a conglomerate in which the pebbles are cemented together 
by lime carbonate, the solution of the cement sets free a considerable 
quantity of gravel, so that a small amount of solution prepares 
a large amount of sediment for removal. A stream might cut its 
valley much more rapidly in such rock than in a compact lime- 
stone, though the latter is, as a whole, the more soluble. 

Structure. The structure of rock has much to do with the rate 
of its erosion. Other things equal, stratified rock is more readily 
eroded than massive rock, since stratification planes are planes of 
cleavage, and therefore of weakness. Taking advantage of these 
planes, the water has less breaking to perform to reduce the material 
to a transportable condition. For the same reason, a thin-bedded 
formation is eroded more easily than a thick-bedded one. 

The beds of stratified rock may be horizontal, vertical, or in- 
clined, and inclined strata may stand at any angle between hori- 
zontality and verticality. In indurated formations the rate of 
erosion is influenced both by the position of the strata and by the 
~ relation of the direction of the flowing water to their dip and strike 
(Chapter X). In general, strata which are horizontal, or but slightly 
inclined, are probably less favorably situated for rapid erosion than 
those which are vertical or inclined at considerable angles. Joints 
have somewhat the effect of bedding planes, so far as erosion 1s con- 
cerned. 

Influence of climate. Climate has both a direct effect on erosion, 
chiefly through precipitation, changes of temperature, and wind; 
and an indirect effect, chiefly through vegetation. Like declivity 
and rock structure, climate does not affect all elements of erosion 
equally. 

Direct effects. The effects of variations of temperature on rock 
weathering have been discussed in Chapter II. Since high tem- 
perature favors chemical action, the weathering of rock by decom- 
position is at its best where the temperature is uniformly high, and 


82 WORK OF RUNNING WATER 


moisture abundant. The climatic conditions favoring chemical 
weathering are therefore different from those favoring mechanical 
weathering (p. 25). 

So long as the water of the surface and that in the soil remain 
unfrozen, temperature affects neither corrasion nor transportation. 
But in middle and high latitudes the surface is frozen for some part 
of each year. During this time corrasion is at a minimum, for 
although the streams continue to flow, there is relatively little 
water running over the surface outside the drainage channels, and 
that little is relatively ineffective. Under some conditions, there- 
fore, temperature affects both corrasion and transportation. 

The humidity of the atmosphere has an influence on the rate of 
erosion. A moist atmosphere favors oxidation, carbonation, hydra- 
tion, and the growth of vegetation, all of which promote certain 
phases of rock weathering. On the other hand, humidity tends to 
prevent sudden and considerable variations in temperature, thus 
checking the weathering effected by this means. Precipitation, 
the most important single factor in determining the rate of erosion, 
is dependent on atmospheric humidity. Its amount, its kind (rain 
or snow), and its distribution in time, are the elements which 
determine its effectiveness in any given place. 

Other things being equal, the greater the amount of precipita- 
tion the more rapid the corrasion and transportation. Much, how- 
ever, depends on its distribution in time. A given amount of 
rainfall may be distributed equally through the year, or it may fall 
during a wet season only. The maximum inequality of distribution 
would occur if all the rainfall of a given period were concentrated 
in a single shower. With such concentration the volume of water 
flowing off over the surface immediately after the downpour 
would be greater than under any other conditions of precipitation, 
and since velocity is increased with volume, and erosive power 
with velocity, it follows that the erosive power of a given amount 
of water would be greatest under these circumstances. Further- 
more the largest proportion of the precipitation would run off over 
the surface under these circumstances, for less of it would sink 
beneath the surface, and less would be evaporated. 

If erosive power and rate of erosion were equal terms, the maxi- 
mum concentration of rainfall would be the condition for greatest 
erosion; but we have seen (p. 79) that erosive power and rate of 
erosion do not always correspond. If the water falling in this way 


EFFECT OF CLIMATE ON EROSION 83 


could get all the material it could carry, erosion would be at a maxi- 
mum; but if the amount of material available for transportation is 
slight, a large part of the force of the water could not be utilized in 
erosion. While, therefore, it is not possible to say what distribu- 
tion of rainfall favors most rapid erosion without knowing the 
nature of the surface on which it is to fall, enough has been said to 
show that the problem is not a simple one. Some of. the most 
striking phases of topography developed by erosion, such as those 
of the Bad Lands (Figs. 68 and 69), are developed where the rain- 
fall is distributed unequally in time, and too slight or too infrequent 
to support abundant vegetation. 

Erosion in arid regions differs from that in regions of abundant 
rainfall in several ways. It is obvious that the valleys will develop 
more slowly in the former, that they will remain young longer, 
that the period necessary for the dissection of the surface is greater, 
that the water-courses will be less numerous, and that fewer of 
them will have permanent streams. If the arid region is high and 
composed of heterogeneous strata, the topography which erosion 
develops is more angular (Fig. 70) than that of the humid region. 
This is because there is less rock decay, and less vegetation to hold 
the products of decay. The more resistant beds of rock, therefore, 
come into greater prominence, especially on slopes, where they 
develop cliffs (Figs. 70 and 73). These general principles find 
abundant illustration in the plateaus of the western part of the 
United States,! where cliffs are by no means confined to the imme- 
diate valleys of the streams. _ 

Indirect effects. Through vegetation, climate influences erosion 
in ways which are easily defined qualitatively, but not quantita- 
tively. Both by its growth (wedge-work of roots) and by its decay 
(supplying COs, etc., to descending waters), vegetation favors cer- 
tain phases of weathering; but, on the other hand, it retards corra- 
sion and transportation both by wind and water. ‘This is well 
shown along the banks of streams and on the faces of cliffs com- 
posed of clay, sand, etc. Its aggregate effect is probably unfavor- 
able to erosion by mechanical means, and favorable to that by 
chemical processes. Winds have much to do with the rate of 
evaporation and the distribution of rainfall, so that their indirect 
effect on erosion is important. 


1Dutton. Tertiary History of the Grand Canyon District, Mono. II, U.S. 
Geol. Surv. 


84 WORK OF RUNNING WATER 


RATE OF DEGRADATION 


The amount of mechanical sediment which the Mississippi 
River carries to the Gulf of Mexico was estimated many years ago 
to represent a rate of degradation for the Mississippi basin of about 
one foot in 5,000 years. But the mechanical sediment carried to 
the Gulf does not really represent the total degradation of the basin, 
for the water which sinks beneath the surface is dissolving more 
or less rock substance, especially lime carbonate. This material is 
carried to the sea in solution, and does not appear in the sediment on 
which the above estimate is based. More recent studies, based on 
fuller data, indicate that the average rate of degradation for the 
United States is about 1 foot in 9,000 years." 

The sediment carried to the Gulf by the Mississippi River is 
gathered from nearly all parts of its basin, but much more of it 
comes from some places than from others. On the whole, the 
rate of erosion is probably greatest toward the margins of the basin, 
where the land is in its topographic youth or early maturity. It is 
notably less in the middle courses of the valleys, and is exceeded by 
deposition in some places along the lower courses of the Mississippi 
and some of its main tributaries. 

The average elevation of North America is probably not far 
from 2,000 feet. If it is being degraded at the rate of one foot in 
9,000 years, and if this rate were to continue, it would take some- 
thing like 18,000,000 years to bring the continent to sea-level. But 
this rate of degradation could not continue to the end, for as the 
continent became lower, the streams would become sluggish and 
erosion less rapid. Long before the continent reached base-level, 
the rate of degradation, so far as dependent on mechanical erosion, 
would become so slow that the time necessary to bring the continent 
to sea-level would be prolonged almost indefinitely. Furthermore, 
it is quite possible that the land is suffering, or is liable to suffer, 
uplift, relative or absolute. If the rate of rise were equal to the 
rate of degradation, the average height of the continent would of 
course not be affected. 


FEATURES RESULTING FROM SPECIAL CONDITIONS OF EROSION 


Running water develops many striking topographic and scenic 
features. Some of them depend primarily on the conditions of 
1 Water Supply Paper 234, U.S. Geol. Surv., pp. 78-83. 


PLATE VII 


f 


Fe Brang 


fey, 


¢ 





Fic. 1—A meandering stream. The Mis- [ic. 2.—A stage in the develop- 


souri River. Scale, about 2 miles per ment of a meander. Schell 
inch. (Marshall, Mo., Sheet, U. S. Geol. River. Scale, about 2 miles 
Surv.) per inch. (Butler, Mo., Sheet, 


U.S. Geol. Surv.) 











| 
ais 


A co 





q] 


























Fig. 3.—A plain in old age. Scale, about 2 miles per inch. Contour in- 
terval, 50 feet. (Abilene, Kan., Sheet, U. S. Geol. Surv.) 


PLATE VIII 






( 


jowl and Mylls 


Cushetunk and Round Mountains, New Jersey; examples of isolated moun- 
tains left by the removal of less resistant surroundings. Scale, about 
‘1 mile per inch. Contour interval, 20 feet. (High Bridge Sheet, U. S, 
Geol. Surv.) 


BAD LANDS 80 


erosion, such as climate, altitude, etc., while others depend largely 
on the structure and resistance of the rock. 

Bad lands. A type of topography developed in early maturity 
in certain high regions where the rock is but slightly, though un- 





Fig. 68. Bad lands of South Dakota. Oligocene formation. (Williston.) 


equally, resistant, is termed bad-land topography (Figs. 68 and 60). 
Bad-land topography is found in various localities in the West, 
but especially in western Nebraska and Wyoming, and the western 





Fig. 69. Bad-land topography north of Scott’s Bluff, Neb. (Darton, U. S. 
Geol. Surv.) — j 


WORK OF RUNNING WATER 


86 


(‘AINS *TO94) °S ‘Q) “SouUOF]) 


"}2] 94} We pey oq 0} SI IBA Oy} Jo asdus y 


‘OpeIojoD 9Y} Jo uoAURD pueID oy} Jo Jed & jo 


yowys "04 “B17 





CANYONS 87 


parts of the Dakotas. Many of the formations here are sandstone 
or shale, alternating with beds of unindurated clay. Climatic factors 
also are concerned in the development of this topography. <A semi- 
arid climate, where the precipitation is much concentrated, seems to 
be most favorable for its development. 

Canyons. Various conditions influence the size and shape of 
valleys, especially in the early stage of their development. High 
altitude favors swiftness of flow, and the development of deep 
valleys. Such valleys will be narrow if the conditions which deter- 
mine widening are absent or unfavorable. An arid climate favors 
the development of narrow valleys if there is sufficient water to 
maintain a vigorous stream, because there is little slope wash. 





Fig. 71. Grand Canyon of the Colorado. (Peabody.) 


Narrow valleys with steep slopes will also be favored if the valley 
is cut in rock which is capable of standing with steep faces. Thus 
a stream may develop a narrow valley in firm rock, where it would 
not do so in loose gravel. Aridity, high altitude, and the proper 
sort of rock structure therefore favor the development of deep narrow 
valleys. Such valleys are canyons, and many of the young valleys 
in the western part of the United States, where these conditions pre- 
vail, belong to this class. 

While all canyons are valleys, most valleys are not canyons. 


88 WORK OF RUNNING WATER 


In popular usage, the rule seems to be that if a valley is sufficiently 
deep, narrow, and steep-sided to be distinctly striking, it is called 
a canyon in regions where that term is in use. Whether a valley 
is deep, narrow, and steep-sided enough to be striking, clearly 
depends on the observer. The Colorado Canyon (Figs. 70 and 71) 
is the greatest canyon known, but it is rarely more than a mile deep, 
and where its depth approaches this figure its width at the top is in 
most places 8, 10, or even 12 miles. Its width at bottom is little 
more than the width of the stream; that is, a few hundred feet. 
Its cross-profile throughout much of its course is therefore not in 
keeping with the conventional idea of a canyon. With a depth 
of one mile and a width of eight, the slope, if uniform, would have 
an angle of less than 15°. Such 
ae a valley is represented in Fig. 
Gana 72. As a matter of fact the 
Fig. 72. Diagram showing the pro- 
portions of a valley the width of which is slopes of a canyon are not com- 
eight times the depth, about the propor- monly uniform, but more like 
tions of the Colorado Canyon. those of Fig. 73. The step-like 
slopes are due to inequalities of hardness. It is perhaps needless 
to say that toan observer on the rim of the canyon, the slopes seem 
several times as steep as those shown in the diagrams. 
Like all valleys which are narrow relative to their depth, the 
Colorado Canyon, great as it is, is a young valley, for it represents 





NNN AN ee 





Fig. 73. Cross-section of the Colorado Canyon. (After Gilbert and Brigham.) 
but a small part of the work which the stream must do to bring 
its drainage basin to base-level. 

While aridity and high altitude are conditions which favor the 
development of canyons, as shown by the fact that most canyons 
are in high and dry regions, they are not indispensable. Niagara 
River has a canyon below its falls, and the surrounding region is 
neither high nor arid. The narrow part of the valley is so young 
that side erosion has not yet widened the valley or lowered its angle 
of slope to such an extent as to destroy its canyon character. This 


canyon is often called a gorge, a term frequently applied to small 
valleys of the canyon type. 


EFFECTS OF UNEQUAL HARDNESS 89 


EFFECTS OF UNEQUAL HARDNESS 
Inequalities of hardness give rise to many peculiarities of topog- 
raphy, and to many scenic features. To this category belong many 
rapids, falls, narrows, terraces, and many striking hills and ridges. 










_ Fig. 74. Diagram illustrating the development of a fall where the hard layer 
dips up-stream. 


Falls and rapids. Falls and rapids are most commonly de- 
veloped where streams pass from more resistant to less resistant 
rock. The greater wear of the 
latter gives origin to rapids. 
At first the rapids are slight (a, 
Fig. 74), but they become more 
considerable (6) as time and 
erosion go on. When the bed 
of the rapids becomes so steep 
that the water falls (as at cd) 
rather than flows over the 
rock surface below the hard 
layer (ha), erosion assumes a 
new phase. The hard layer is 
then undermined (Fig. 76), and 
the undermining causes the 
falls to recede. This phase of 
erosion is sometimes called 
sapping. 

If the hard layer which 
occasions a fall dips up-stream 
(Fig. 74), its outcrop in the 
stream’s bed becomes lower as 
the fall recedes. When it has 
become so low that the water 
passing over it no longer reacts 
effectively against the less resistant material beneath, sapping 
ceases, and the fall is then transformed again into rapids. The 
history of rapids which succeed falls is the reverse of the history 





Fig. 75. Lower fall of the Yellowstone. 


go WORK OF RUNNING WATER 


which preceded. The later rapids are steepest at the beginning of 
their history, the earlier at their end. Stated in other terms, rapids 
are steepest when nearest falls 
in time. Slight differences in 
the resistance of successive 
layers may occasion successive 
falls or rapids (Fig. 77). 

If the layers of unequal 
hardness in a stream’s bed are 
vertical and the course of the 
stream at right angles to the 
strike, rapids, and perhaps 
falls, will develop. Such falls 
would not recede. 

The inequality of resistance 
in the rock which occasions a fall may be original or secondary. In 
the case of Niagara Falls (Fig. 76) relatively resistant limestone 
overlies relatively weak shale. 
At the Falls of St. Anthony 
(Minneapolis) limestone over- 
lies friable sandstone. The 
falls of the Yellowstone are in 
igneous rock. In this case the 
unequal resistance is caused by 
unequal decay of the rock, due 
perhaps to the rise of hot va- 
pors| which have decomposed 
and weakened the rock in the 
areas through which they have 
ascended. Such action is com- 
mon in volcanic regions. 

One waterfall may breed 
others. ‘Thus where a fall 
recedes beyond the mouth of a 
tributary stream, the tributary 
falls. The Fall of Minnehaha 
creek tributary to the Missis- Fig. 77. Bridal Veil Fall, Kamloops, 

oe be : SP es British Columbia. 

sipp1 near Minneapolis, is an 

_ illustration. Once in existence, the fall of a tributary follows the 
same history as that of a main stream. 











































































































Fig. 76. Diagram illustrating the con- 
ditions at Niagara. (Gilbert.) 





EFFECTS OF UNEQUAL HARDNESS — gr 


The fall of the Niagara! is one of the most remarkable known 
both because of its large volume of water and its great descent, 


between 160 and 170 feet. This fall is 
divided into two parts, separated by 
Goat Island, the Horseshoe fall (Fig. 79) 
on the west, and the American fall on 
the east. Between 1842 and 1005, the 
Horseshoe fall receded about five feet 
a year, while between 1827 and 1005, 
the American fall receded less than 
three inches a year.! 

Rock terraces. Where a hard layer 
outcrops in the side of a valley above 
its bottom, the side slopes of the valley 
become gentle just above the hard layer, 
and steep, or even vertical, at and be- 
low its outcrops, as illustrated by Fig. 
80. The hard layer, H, then stands out 
as a rock terrace on either side of the 
valley. Such terraces are not rare, and 
are popularly velieved to be old ‘‘ water- 





Fig. 78. Twin Falls, Yoho 
Valley, British Columbia. 


lines”; that is, to represent the height at which the water once 
stood. In one sense this interpretation is correct, since a river has 








Fig..79. Niagara Falls. (U.S. Geol. Surv.) 
1 Gilbert, in Physiography of the United States, and Bull. 306, U.S. Geol. Surv. 


Q2 WORK OF RUNNING WATER. 


stood at all levels between that of the surface in which its valley 
started, and its present channel; but the shelf of hard rock does 
not mean that the river was ever so large as to fill the valley 
from its present channel to the level of the terrace. Rock terraces 
may also result from changes of level. 

Narrows. Where a stream crosses vertical or highly inclined 
strata of unequal resistance, its valley is usually constricted at the 
crossing of the hard layers. If such a constriction is notable it is 
called a narrows, or sometimes a water-gap (Fig. 81). The Appa- 
lachian Mountains afford nu- 
merous examples. The nar- 
rows develop because the 
processes which widen the 
valley are less effective on 
more resistant rock than on 
less resistant. Some narrows 
arise in other ways also. 

Narrows are much more conspicuous in certain stages of erosion 
than in others. While a valley is still so young as to be narrow at 
all points, there can be no pronounced ‘‘narrows”’; but later, when 
the valley is elsewhere wide, narrows become pronounced. From 
what has preceded it is clear that rapids or falls are likely to occur 
at narrows, especially early in their history. 

Other effects. Inequalities in the hardness of rock develop 
certain peculiarities of topography outside of valleys. The less 





Fig. 80. Rock terraces due to a resist- 
ant layer, H, of rock. 








Fig. 81. The Kittatinny Mountain and Delaware ‘Water-Gap from Manunka 
Chunk. (N. J. Geol. Surv.) 


EFFECTS OF UNEQUAL HARDNESS 93 


resistant portions of a land area more or less distant from streams 
are worn down more readily than those which are more resistant. 
If great areas of high land are capped with hard rock, they are likely 
to remain high after surrounding areas of less resistance are brought 
low. If the hard capping remains over a small area instead of a 





Fig. 82. The Enchanted Mesa. A striking butte in New Mexico. The name 


mesa is not commonly applied to elevations of such small summit area. (R. T. 
Chamberlin.) 


large one, the elevation is a butte, a hill, or a mountain; if over a 
large area, a plateau. Many buttes and small mesas are but 
remnants of former plateaus. A feature of buttes and mesas capped 





Fig. 83. Hogbacks; Sec. 22, T. 16, N., R. 112 W., Wyoming. The rock which 
occasions the hogbacks is the Lazeart sandstone at the base of the Adaville forma- 
tion. (Veatch, U.S. Geol. Surv.) 


94 WORK OF RUNNING WATER 


by hard rock is the steep slopes or cliffs corresponding to the edges 
of the hard beds (Figs. 70 and 82). 

If the rock of a region is stratified and the layers tilted, the 
removal of the softer beds leaves the harder ones projecting above 
the general level in the form of ridges or ‘‘hog-backs” (Fig. 83). 
Dikes of igneous rock, harder than the beds which they intersect, 
likewise become ridges after the degradation of their surroundings. 
The plugs of old volcanic vents and other igneous intrusions of 











Fig. 84. A monadnock; a mass of intruded igneous rock isolated by erosion, 
and remaining high because of its superior hardness. Matteo Tepee, Wyo. (De- 
troit Photo. Co.) 


limited area may constitute conspicuous hills or mountains (Fig. 
84) after erosion has removed their less resistant surroundings. 
Cushetunk Mountain, Pl. VIII, is an example. 

Ridges and hills resulting from the unequal degradation of 
unequally resistant strata are most prominent in the late maturity 
or early old age of an erosion cycle. The outcropping masses of 
hard rock are then more perfectly isolated than at earlier stages. 
Most of the even-crested ridges of the Appalachian system, as well 
as many others which might be mentioned, became ridges in this 


EFFECTS OF UNEQUAL HARDNESS 95 


way. In the final stages of an erosion cycle, the ridges of hard 
rock are themselves brought low. Isolated remnants of hard rock 
which remain distinctly above their surroundings in the late stages 
of an erosion cycle are known as monadnocks, the name being derived 
from Mount Monadnock, N. H., an elevation of this sort developed 
in a cycle antedating the present. 


THE EROSION OF FOLDS 


The erosion of folded strata (anticlines and synclines) leads to 
the development of distinctive topographic features. So soon as 
a fold begins to be lifted, it is, by reason of its position, subject to 
more rapid erosion than its surroundings. For the same reason, 
the crest of a fold is likely to be degraded more rapidly than its 
lower slopes, and must suffer more degradation before it is brought 
to base-level. Most folds are composed of beds of unequal resist- 
ance, and as their degradation proceeds, successive layers are worn 
from the top, and the alternating layers of more and less resistant 
rock are exposed. The less resistant beds are worn down faster 
than the others, and in time the outcrops of the stronger beds 
become ridges, distinctly above the outcrops of the weaker beds 
which have become valleys and lowlands (Fig. 85). 


SSYOr ~S = 





Fig. 85. A canoe-shaped valley bordered by a ridge formed by the outcrop of 
a hard layer in a plunging syncline. The ridge bounding the canoe-valley is 
separated from an outer ridge by a curved valley, underlain by relatively weak 
sock. (Willis.) 


g6 WORK OF RUNNING WATER 


If the axis of an eroded anticline were horizontal, a given hard 
layer, the arch of which has been cut off, would outcrop. on both 
sides of the axis. When the topography has become mature, these 
outcrops will constitute parallel ridges, or parallel lines of hills. 
When the region had been base-leveled, the outcrop will be in 
parallel belts, though no longer ridges or hills. The lower the 
plane of truncation, the farther apart the outcrops will be in the 


oe ae arnt mannan, 
ei os vie oe. ae 
f Ss a We vith oR ~ 
‘. f "pe 


4 
\ 
ine ‘ mere Sone \ 
7 ¥ ‘, 
gx : Moe 2 DN ANG 
os * 


. 
-—_— =» . ee ee 
* ‘ f ‘ 







“S007 





Fig. 86. Diagram showing the outcrops of hard layers (shaded) on the flanks 
of truncated folds; cd, present surface; ab, an earlier erosion surface. 


anticline, and the nearer together in the syncline (compare outcrop 
of H, along ad and cd, Fig. 86). 

If, on the other hand, the axis of the anticline or syncline is not 
horizontal, that is, if it plunges (dips), the topographic result will 
be different. In this case the outcrops of a given layer on opposite 
sides of an anticline will converge in the direction of plunge, and 
come together. At a stage of erosion antedating planation (say 
late maturity) there will be a ridge or a succession of hills, in the 
position corresponding to the outcrop of a hard layer, with a canoe- 
shaped valley within. If two hard layers are involved, instead 
of one, there will be two encircling ridges, with a curved valley 
between them, and a canoe-shaped valley within the innermost 
(Fig. 85). A succession of plunging anticlines and synclines might 
give rise to a very complex series of ridges and valleys. Illustra- 
tions of the above phenomena are found at various points in the 
Appalachian Mountains.’ | 






















ED — > = 277 PD eee . 7 PUP =< SS 


SS 


PQS 


Fig. 87. Cross-section of a portion of the Appalachian Mountains to illustrate 
the relations of mountain ridges to anticlines and synclines, and the phenomena of 
erosion cycles. (Rogers.) 


1 Willis, The Northern Appalachians, in Physiography of the United States. 


STRUCTURAL ADJUSTMENT 97 


In the structural adjustment which goes with the erosion of 
tolds, it happens in many cases that valleys come to be located on the 
anticlines after the latter have been worn down, while the outcrops 
of the hard layers on the flanks of the anticlines, or even in the 
original synclines, become the mountains (Fig. 87). 


ADJUSTMENT OF STREAMS TO ROCK STRUCTURES 


Valleys (gullies) are located at the outset without immediate 
regard to the hardness and softness of their beds. It is primarily 
che slope about the head of a gully which determines its line of 
growth, and, once established, streams tend to hold their courses; 
but the streams on the weaker rock will deepen their valleys more 
rapidly than others, and have an advantage over them. Being 
deeper, their tributaries may be lengthened until their heads reach 
the other valleys, with the results shown in Figs. 88-90. Even 
where several streams cross the same resistant bed, piracy is likely 
to take place among them, for some are sure to deepen their valleys 
faster than others, because of inequalities of volume, load, or hard- 
ness. This is illustrated by Figs. 91-93. (See also Figs. 52 and 53.) 
Piracy may take place where streams do not flow over rock of un- 
equal resistance, but it is more common where they do, for greater 
resistance of rock puts the stream which crosses it at a disadvantage 
as compared with the stream which crosses less resistant rock. 

The changes in the courses of streams by means of which they 
come to sustain definite and stable relations to the rock structure 
beneath, are known as processes of adjustment.' Since streams 
and valleys adjust themselves to other conditions as well, this 
phase of adjustment may be called structural adjustment. Struc- 
tural adjustment is not uncommon among rivers flowing over strata 
which are vertical or highly inclined, since in these positions, strata 
of unequal resistance are most likely to alternate with one another 
at the surface. The processes of adjustment go on until the streams 
flow as much as possible on the weaker beds, and as little as possible 
on the stronger. Adjustment is then complete. This amounts to 
the same thing as saying that the outcrops of resistant layers 
tend to become divides. In many cases an area is so situated that 
there is no escape for its drainage except across resistant rock. In 
tnis case drainage is completely adjusted when as few streams as 
possible cross the resistant rock, and these by the shortest routes. 


1 Campbell, Jour. Geol., Vol. IV., pp. 567, 657. 


98 WORK’ OF RUNNING WATER. 


Adjustment has been carried to a high degree of perfection in 
many parts of the Appalachian system. Here, as in all other 
mountains of similar structure, strata of unequal hardness were 
iolded into ridges. The folds were then truncated by erosion, 





SUL EO MEE OLDT 6S! VG 
4 apie: 


- 





Figs. 88-90. Figs. 91-93. 
Figs. 88-90. Diagrams illustrating piracy, where the stream which does not 
flow over rock of superior hardness captures those which do. Fig. 89 represents a 


iurther development of the drainage shown in Fig. 88, and Fig. go represents a still 
later stage. 


Figs. 91-93. Diagrams to illustrate piracy where the competing streams all 
cross a hard layer. The diagrams represent successive stages of development. 


exposing the more and the less resistant beds (H and S, Fig. 86, 
respectively) in alternate belts along the flanks of the truncated 
folds (truncated at ab and cd). The streams, especially the lesser 
ones, now flow along the strike of the weaker beds much more 


INFLUENCE OF JOINTS 99 


commonly than elsewhere, and where they cross the hard layers it 
is in most cases at right angles to the strike. This is shown in Fig. 
94, where the arrows indicate a “10° 
the direction of strike. s 

As base-level is approached, 
the outcrops of hard rock are 
brought low. When the resist- 
ant beds have been reduced to 
base-level, streams may flow 
without regard to the resistance 
of the rock beneath, for down- 
ward cutting has ceased. 

It happens in some cases eh 
that rocks of unequal resistance 


are covered by beds of uniform 
hardness. A consequent stream 
(p. 61) developed on the latter 
37°00 


may find itself out of structural 


adjustment when its channel Fig. 94. Adjusted drainage in a region 
is sunk to the level of the of folded rocks. The many nearly paral- 


heterogeneous beds Bratch lel streams are flowing with the strike. 
a stream is said to be superimposed (Fig. 95) on the underlying 
structure. Structural adjustment is likely to follow in time. 





INFLUENCE OF JOINTS ON EROSION 


It has been pointed 
out that joint planes have 
somewhat the same influ- 
ence upon erosion that 
bedding planes have when 
the beds are tilted at a 
high angle. Most rocks 
are affected by joints, 
and many of them are 
nearly vertical. Two sets 
are generally present, and 
in some places more. When 


there are but two, they Fig. 95. Diagram to illustrate superimposition. 
The consequent stream on the upper formation 
usually meet at a large was superimposed on the underlying structures 


angle (Fig. 2). These when the upper bed had been cut through. 





100 WORK OF RUNNING WATER 





Fig. 96. Figure showing crenate river bank, the re-entrants being determined 
by joints. Dells of the Wisconsin River, near Kilbourn, Wis. (Atwood.) 


joints allow the ingress of water, roots, etc., which help to weather 
and disrupt rocks. Their effect on erosion may be seen along many 
streams which flow in rock gorges. In such situations, the outlines of 
the banks are in some cases angular, and in some crenate (Fig. 96), 
the re-entrants being located at the joints. By working into and 
widening joints, running water in some places isolates masses of 
rock as islands (Fig. 97). 





Fig. 97. An island formed by river erosion in jointed rock; Lower Dells of the 
Wisconsin. (Atwood.) 


PLATE IX 





Fic. 1.—Entrenched Meanders. 
terval, 20 feet. 


Scale, about 1 mile per inch. Contour in- 
(Harrisburg, Pa., Sheet, U. S. Geol. Surv.) 


faect>, 
=. 


»> 
Ling 


Los Angeles 


‘\oJunetion 
oa 


» 
\ 
" 

w 
ny 
¥ 
w 
ny 
ny 


Pl Lae 


Fa? = 


“ = 
*. eee 
+4277 
ise # 
— 

a) 

Fee. 


. 


ON sf 


Pecrecse sss. 
s 





Oceanside “G 
Fic. 


&. KY 


2.—A Section of the California Coast, showing lands near the coast, 
which have recently emerged. Scale, about 1 mile per inch. Contour 
interval, 20 feet. (Oceanside, Cal., Sheet, U. S. Geol. Surv.) 





aR 
5 
DN 
— 
2° 
o 
o 
7) 
2) 
prey 
ro) 
rob) 
a 
QD 
aS 
S&S 
oO 
t 
oo 
=| 
=) 
= 
3S 
oO 
ss 
eat 
cS) 
t= 
a 
o 
jor 
) 
ura 
= 
re 
~Y 
= 
e) 
Ke 
ins 
o 
Lead 
3 
oO 
NM 


A piedmont alluvial plain or compound alluvial fan in Southern California. 


CHANGES OF LEVEL IOI 


In a region free from mantle rock, or where the mantle rock is 
meagre, joints have determined the courses of many valleys by 
directing the course of surface drainage. This is well shown in 
many parts of the arid West. In regions where the rocks are faulted 
the courses of some streams are controlled by the faults. It is 
probable that joints and fault planes have been more important 
in locating valleys, especially where the mantle rock is thin, than 
was formerly recognized. 

Joints in rocks may occasion the development of natural bridges. 
If above a waterfall, for example, there is an open joint in the 
bed of the stream 
(as at b, Fig. 98), 
some portion of 
the water will de- Rrkeh GEEMbN cer tes ence 

; Diagram to illustrate the initial stage in the 
scend through it. development of a natural bridge. Longitudinal section 
After reaching a at the left, cross-section at the right. 
lower level it may find or make a passage through the rock to the 
river at the falls. If even alittle water takes such a course, the flow 
will enlarge the 
passageway 
through the joint 


to the valley at ae poten ta : 
the falls (bcde, Fig. 99. A stage later than that shown in Fig. 98. 









Fig. 98). This passageway may in time become large enough to 
accommodate all the water of the river. The entire fall will then 
_be transferred from the position which it previously occupied (f) 
to the position of the enlarged joint (6). The fall will then recede. 
The underground channel between the old falls and the new will 
then be bridged by rock (bf” and f’”’, Fig. 99). The natural 
bridge near Lexington, Va. (Fig. 100), almost 200 feet above the 
stream which flows beneath it, is believed to have been developed 
in this way. It is not to be understood that all natural bridges ! 
have had this history. 


EFFECT OF CHANGES OF LEVEL 


Rise. If, after being base-leveled, or notably advanced in an 
erosion cycle, a region is uplifted so as to increase the gradients 
and velocities of its streams, they are said to be rejuvenated. 
Renewed youth differs from first youth, in that the streams 

+ Cleland, Pop. Sci. Mo., May ’11, and Bull. G. S. A., Vol. XXT, p. 313. 


I02 


are already in existence. 


Fig. 
Virginia. 


(U. S. Geol. Surv.) 


is, the old meanders are entrenched. 





WORK OF RUNNING WATER 


The rejuvenated streams erode their 
valleys after the manner of youthful streams. 


They excavate new 
valleys in the bottoms of older 
ones (Figs. tor and 102), deep- 
ening them until they reach the 
new grade plane. Young val- 
leys in the bottoms of old ones 
are one of the evidences of re- 
juvenation. The new valley in 
the old one may be developing 
all along its course at the same 
time, or it may begin at the de- 
bouchure of a stream and work 
headward. In either case, the 
tributaries are rejuvenated when 
their main is lowered at the 
point of union. 

Another evidence of rejuve- 
nation is found in entrenched 
meanders. When an old wind- 
ing stream is rejuvenated, the 
deepened channel follows the 
course of the stream before re- 
juvenation. The result is that 
a new winding gorge is cut; that 
Entrenched meanders are 


rather common in the Appalachian Mountains (Fig. 1, P]. 1X), and 


are known in other parts of the 
world.!. With rejuvenation of 
the drainage, a new cycle of ero- 
sion is begun, whether the pre- 
ceding one was complete or not. 

The principles involved in 
the recognition of cycles of ero- 
sion, separated by uplifts, are 





Fig. 1o1. Cross-section of a wide 
valley, ab, in the bottom of which a 
younger valley, cd, has been excavated, 
as the result of uplift. 


illustrated by Fig. 103, which represents an ideal profile of consider- 
able length (say 20 miles). The points a, a’, and a” have about 


the same elevation. 


Below them there are areas 0, 0’, and 6’, which 


1 Davis. The Seine, the Meuse, and the Moselle. Nat. Geog. Mag., Vol. VII, 


pp. 181-202, and 228-238. 


CHANGES OF LEVEL 103 


have a nearly common elevation, below which are the sharp 
valleys d, d’, andd’’. The points a, a’, and a” represent the tops of 
ridges formed by the outcrops of layers of hardrock. If the crests 
of the ridges are level, the points 
a, a’,and a” must represent remnants 
of an old base-level, sznce at no time 
after a ridge of hard rock becomes 
deeply notched does it acquire an even 
crest, until it is base-leveled.1 After 
the cycle represented by the rem- 
nants a, a’, and a’’ was completed, 
the region suffered uplift. A new 
cycle represented by the plain 3, 0’, 
and 6’ was well advanced, though 
not completed, when the region was 
again elevated, and the rejuvenated 
streams began to cut their valleys d, 
d’, and d” in the plain of the previous 
incomplete cycle. The elevations, ¢ 
and c’ (intermediate between a, a’, 
4} / /} 

and a”, and 6, 6’, 6”), may represent Fig. 102. Diagram to illustrate 
either remnants of the first base-level an ideal case of rejuvenation as the 
plain, lowered but not completely result of uplift. The black area 
removed while the plain 8, 0’, b” was 2 the bottom represents the sea. 

developing; or they may represent a cycle intermediate between 


that during which a, a’, a’ and 3, b’, b’’ were developed. 
x 








Fig. 103. Diagram to illustrate cycles of erosion where the beds are tilted. 


If the strata involved are horizontal, the determination of 
cycles may be less easy. . Thus in Fig. 104, it is not possible to say 
whether a and a’ represent remnants of an old base-level, or whether 
they represent the original surface from which degradation started. 
So, too, the various benches below a, such as 3, b’, and 6’, might 
well be the result of the superior hardness of beds at this level. 
For the determination of successive cycles in the field, it is necessary 


1 Other views have been entertained. See Tarr, Am. Geol., Vol. XXI, pp. 351- 
370, and Daly, Jour. of Geol., Vol. XIII, pp. 105-125. 


104 WORK OF RUNNING WATER 


to consider areas of considerable size, and to eliminate the topo- 
graphic effects of inequalities of hardness. 

It is by the application of the preceding principles that it is 
known that the Appalachian Mountains, after being folded, were 
reduced to a peneplain (the Kittatinny peneplain) from the Hudson 
River to Alabama. The old peneplain surface is indicated by 





Fig. 104. Diagram to illustrate cycles of erosion where the beds are horizontal. 


the level crests of the Appalachian ridges. The system was then 
warped (not folded) up, and in the cycle of erosion which followed, 
broad plains were developed at a new and lower level, corresponding 
in a general way to the plains 0, 6’, and 6” of Fig. 103. The plains 
were located, for the most part, where the less resistant strata come 
to the surface. Above them rise even-crested ridges, the outcrops 
of the resistant layers, isolated by the degradation of the weaker 
beds between. It is the outcrops of these layers which constitute 
many of the present mountain ridges corresponding to the high 
points of Fig. 103. The evenness of their crests testifies to the com- 
pleteness of the first peneplanation. The evenness of the crests is, 
however, interrupted (1) by notches cut by the streams in later 
cycles, and (2) by occasional elevations (monadnocks) above the 
common level. Most of the monadnocks are rather inconspic- 
uous, but there is a notable group of them in North Carolina 
and Tennessee, of which Mount Mitchell and Roan Mountain are 
examples. When long distances are considered, the ridge crests 
depart somewhat from horizontality. This is believed to be due, 
in part at least, to deformation of the old peneplain during the uplift 
which inaugurated the second cycle of erosion. 

The extent to which the second cycle of erosion recorded in the 
present topography had proceeded before its interruption by up- 
warp is indicated by the extent of the valley plains (Fig. 103) 
below the mountain ridges. While these plains were being devel- 
oped on the weak rocks, narrow valleys (water-gaps) only were cut 
in the resistant rocks which stood out as ridges. Similar valleys, 
whether shallow or deep, from which drainage has been diverted, | 
are sometimes called wind-gaps. 

The second cycle of erosion, still incomplete, was interrupted by 


CHANGES OF LEVEL 105 


uplift (relative or absolute), and a third cycle was inaugurated. 
The third cycle began so recently that it has not yet advanced far. 

Some of the features just described are illustrated by Fig. 8r. 
The even mountain crest in the background is the Kittatinny 
Mountain of New Jersey and its continuation in Pennsylvania. 
In common with other corresponding crests, it is a remnant of the 
oldest recorded base-level (or peneplain) of the region. Below the 
mountain crest there is another plain, developed in a subsequent 
cycle of erosion, while the valley plain in the foreground represents 
the work of a still later cycle. 

Many of the peculiarities: of the drainage of the Appalachian 
Mountain system are intimately connected with the history just 
outlined. Thus three great rivers, the Delaware, the Susquehanna, 
and the Potomac, have their sources west of: the Appalachians 
‘proper, cross the system in apparent disregard of the structure, 
and flow into the Atlantic. The James and Roanoke head far to 
the west, although not beyond the mountain system, and flow east- 
ward, while the New River (leading to the Kanawha) farther south, 
heads east of the mountain-folds, and flows northwestward across 
the alternating hard and soft beds of the whole Appalachian system, 
to the Ohio. The French Broad, a tributary to the Tennessee, has 
a similar course. Such streams are clearly not in structural adjust- 
ment, and afford good opportunities for piracy. Their courses were 
apparently assumed during the time of the Kittatinny peneplain, 
when the streams had so low a gradient as not to be affected by 
structure. Elevation rejuvenated them, and they have held their 
courses in succeeding cycles across beds of unequal resistance, 
though smaller streams have become somewhat thoroughly adjusted. 
Crustal deformations have also helped them to hold their courses, 
for the peneplain seems to have been tilted to the southeast at its 
northern end, and to the southwest at its southern, when the suc- 
ceeding cycle began. | 

Streams which hold their early courses in spite of changes which 
have taken place since their courses were assumed are said to be 
antecedent. They antedate the crustal movements which, but for 
pre-existent streams, would have given origin to a different arrange- 
ment of river courses. As a result of crustal movements, therefore, 
a consequent stream may become antecedent. Master streams are 
more likely to hold their courses, and therefore to become ante- 
cedent, than subordinate ones. 


. 


106 WORK OF RUNNING WATER 


The uplift of base-leveled beds, especially if the beds are tilted 
so as to bring layers of unequal resistance to the surface at frequent 
intervals, affords conditions favorable for extensive adjustment. 
The numerous wind-gaps in the mountain ridges, representing the 
abandoned courses of minor streams, and the less numerous water- 
gaps, which indicate the resistance of large streams to structural 
adjustment, are instructive witnesses of the extent to which ad- 
justment has gone. So extensive has it been among the streams of 
the Appalachian Mountains that there is probably no considerable 
stream in the whole system which has not gained or lost through its 
own or its neighbors’ piracy. 

Sinking. The land on which a river system is developed may 
be depressed relative to sea-level. In this case the sea occupies 
the lower ends of valleys, converting them into bays and estu- 
aries. A valley in this condition is said to be drowned. Of drowned 
valleys there are many examples along the Atlantic coast. Thus 





Fig. 105 Fig. 106 


Fig. 105. Chesapeake Bay and its surroundings. ‘The bay is a drowned river 
valley, and the lower ends of its tributary valleys are also drowned. 

Fig. 106. The drainage of the region about Chesapeake Bay as it would have 
been but for drowning. 


CHANGES OF LEVEL 107 


the St. Lawrence is drowned up to Montreal, and the Hudson up 
to Albany. If the drowned portion of the latter valley were not 
so narrow, it would be a bay. Delaware and Chesapeake bays, 
as well as many smaller ones, both north and south, are likewise 
the drowned ends of river valleys (Figs. 105 and 106). 

Successive rising and sinking. Another peculiarity of valleys 
and streams resulting from changes of level is illustrated by Pl. IX, 
Fig. 2. The main valleys of this part of the coast were developed 
when the land stood higher than now. Later, the sinking of the 
coast converted the lower ends of the valleys into bays. The 
bays were then transformed into lakes or lagoons by deposition at 
their mouths. Subsequent rise of the land or sinking of the sea al- 
lowed the drainage from the lakes to cut across the deposits which had 
converted the bays into lakes. The result is an older, wider valley 
above, succeeded by a younger one near the debouchure. 

Differential movement. Warping. A land surface on which 
a river system is established may suffer warping, some parts going 
up and others down. Above an upwarp which notably checks its 
flow, a stream is ponded. If a stream holds its course across a 
notable uplift athwart its valley, it becomes an antecedent stream. 
The Columbia River has been thought to hold its antecedent 
course across areas which have been uplifted (differentially) hun- 
dreds and even thousands of feet.1 A lesser stream would have been 
diverted, as many of its tributaries have been. 


AGGRADATIONAL WORK OF RUNNING WATER 


We have seen that rivers carry mud, sand, gravel, etc., from 
land to sea, and that their goal is the degradation of the land to 
base-level. We have seen also that rivers do not always carry their 
sediment directly to the sea. In many cases it is dropped for a 
time on land, perhaps to be picked up and carried on again when 
conditions for its transportation are more favorable. We have now 
to inquire more particularly into the causes and results of deposition. 

Causes of deposition. When running water drops its load, or 
any part of it, it is generally because the current has lost velocity. 
Decrease of gradient is the commonest cause of loss of velocity. 
The loss may be (1) sudden, as when the water passes from a steep 
slope to a gentle one, or into a body of standing water; or (2) slow, 
as in following a valley whose gradient decreases gradually. We 

1 Russell. Rivers of North America, p. 279. 


108 WORK OF RUNNING WATER 


therefore look to the places where these changes in velocity occur 
for the principal deposits of running water. Streams also become 
slower wherever their channels become wider, even if volumes and 
gradients remain constant. 

Decrease of volume is a less common cause of decrease of velocity. 
Most streams increase in size as they flow, but to this general rule 
there are exceptions. (1) If a stream flows through a very dry re- 
gion it receives few tributaries, while evaporation is great and the 
thirsty soil and_ rock 
through which it flows 
absorb some of its water. 
In such a region a stream 
may diminish as it flows, 
and may even disappear 
altogether (Pls. II and X). 
(2) In some places certain 
streams break up _ into 


LAKE ST. CLAIR several (Fig. 107), and in 


therefore the velocity of 
each is less than that of 
the original stream. (3) 
<r Many streams, especially 

Fig. 107. Delta mr Lake St. Clair. (Lake in arid regions, have much 
Survey Chart.) of their water withdrawn 
for irrigation. (4) During the decline of their floods, all streams 
decrease in volume and velocity. 





Location and Forms of Alluvial Deposits 


1. At bases of steep slopes. The water of every shower 
washes sediment down the slopes of hills, and much of it is left at 
their bases. Its lodgment there, even where there are no valleys 
or gullies, is shown in some places by the burial of fences by the mud 
lodged against them. ‘Temporary streams flowing down steep 
slopes are checked suddenly at their bases, and abandon there their 
heavy loads of debris. Thus, at the lower end of the new-made 
gully on the hillside there is commonly a mass of detritus which 
was washed out of the gully itself (Figs. 40 and 108). Material in 
such positions accumulates in the form of a partial cone, known as 
an alluvial cone. Alluvial cones have much in common with cones 


this case the volume and > 


—_— 


ALLUVIAL DEPOSITS 109 





Fig. 108. An alluvial cone. (U.S. Geol. Surv.) 


of talus. In the latter, gravity brings the material down with little 
aid from water, but between the two types of cones there are all 
gradations. 

Conspicuous alluvial cones are common at the bases of steep 
slopes in semi-arid regions. The rainfall there is fitful, and the 





Fig. 109. Deposition at the bases of valley slopes, tending to give the valley a 
U-shaped base. Unaweep Canyon, Colorado. (Cross, U. S. Geol. Surv.) 


occasional heavy showers, which give rise to temporary and power- 
ful torrents, favor the development of great cones. At the bases 
of the mountain ranges in the Great Basin, some of the talus and 
alluvial cones are 2,000 or 3,000 feet high. 

An alluvial fan is the same as an alluvial cone, except that it 


110 WORK OF RUNNING WATER 


has a lower angle of slope. The term fav is more appropriate than 
cone for most alluvial accumulations at the bases of slopes. The 
lower angle of the fan may be due to the less abrupt change of 
slope where it is developed, to the larger quantity of water con- 
cerned in its deposition, to the smaller amount of detritus, or to 
its greater fineness. Less change of slope, more water, and less 
and finer material, all favor the wider distribution of the sediment, 
and so the development of fans rather than cones. Nearly all 
young rivers descending from mountains build fans where they 
leave the mountains. Thus, the rivers descending from the Sierras 
to the great valley of California build great fans at the base of the 
range. Many rivers descending from the Rockies to the Great 
Plains have done the same thing. The fans of some streams 
descending from the mountains are many miles across. That 
of the Merced River in California, for example, has a radius of about 
40 miles. 

The fans made by neighboring streams may spread laterally 
until they merge. The union of such fans makes a compound 
alluvial fan, or a piedmont alluvial plain (P|. X). Such plains exist 
at the bases of most considerable mountain ranges. Sheet wash, 
as well as streams, contributes to them. The depth of alluvial 
material in such plains is, in some cases, hundreds of feet. The 
great spread of these land deposits is remarkable. East of the 
Rocky Mountains they extend out more than a hundred miles in 
some places. This wide spread appears to be the result of the long- 
continued action of running water. The cone or fan, as first built, 





i ee 
CLP MP LE LP BP GE LE BPR a Se 2 


Fig. 110. Diagram to illustrate the spreading of alluvial deposits in a piedmont 
position. The deposits may first take the position represented by the line 1-1’. 
At a later stage, as a result of erosion and redeposition, they take the position repre- 
sented by the line 2-2’, being spread farther from the mountain and having a lower 
surface slope. At a still later time, they take the position 3-3’, with a still lower 
slope and a still wider spread. 


ALLUVIAL DEPOSITS III 


is degraded later, and its materials spread more widely, as suggested 
by Fig. r1o. 

Deposits of this sort have probably been far more important in 
the past than has been generally recognized. Much of the material 
of the Coastal Plain of the Atlantic and Gulf slopes of the United 
States appears to have been deposited in this way. A large part 
of the Great Plains is covered with wash from the Rocky Moun- 
tains, and similar deposits are of great extent and depth east of the 
Andes and south of the Himalayas. They are, indeed, of signifi- 
cant extent and depth on the plains about almost every mountain 
range which has been carefully studied. It seems clear that similar 
deposits must have been made at all stages in the past history of 
the earth, whenever and wherever mountainous lands bordered 
plains. 

Formations of this general sort, made at the bases of high lands, 
have now been recognized among the ancient formations of the 





Fig. 111. A branching stream. Junction of the Cooper and Yukon rivers, 
Alaska. Shows also bars, etc. (U.S. Geol. Surv.) 


earth, as well as among the recent ones, and some of the ancient 
beds of sediment deposited in this way attained thicknesses of 
hundreds and even thousands of feet. They probably attained their 
greatest thickness, as now, in basins. 

2. In valley bottoms. A stream which makes deposits in 
its channel, makes the channel smaller. In time it may become 
too small to hold all the water. A part then breaks out, and follows 


II2 WORK OF RUNNING WATER 


a new course over the valley flat. This process may be repeated 
again and again (Fig. 111). Some streams deposit bars in their 
channels, especially in low water. The bars may be swept away 
in time of flood, but some of them become more or less permanent 
islands. 

The profiles of the bottoms of most valleys are curves, the curva- 
ture becoming less as the lower end of the stream is approached 


NOR 
cay MAL VALLEy PROFILE 
EVEL 
SEA LE SEA 


Fig. 112. Profile of a normal valley, showing decreasing slope down stream. 


(Fig. 112). It therefore happens that as a stream descends its 
valley it generally reaches a point where its reduced gradient so 
diminishes its velocity that it must abandon some of its load. In 
this way sediment is distributed for long distances along valley bot- 
toms. It is left in the channels of streams in low water, and spread 
——] over their flood plains in high 
water, aggrading them and 
making them alluvial plains. 
Deposition in a valley which 
has no flat tends to develop one 
(Fig. 113). Alluvial deposits 
Fig. 113. Flat developed by aggrada- On valley flats are ‘usually but 
tion — diagrammatic. a few feet, or at most a few 
scores of feet thick; but in rare cases they reach hundreds of feet. 
Natural levees (Fig. 114) are developed on flood plains aggraded 
by occasional floods. At such times the current in the main chan- 
nel is swift; but as the water escapes its channel and spreads over 





Fig. 114. Levees of the Mississippi in cross-section, four miles north of Donald- 
sonville, La. Vertical scale X50. The horizontal line represents sea-level. The 
bottom of the channel is far below sea-level at this point. 


the adjacent flat, its velocity is checked promptly, because its 

depth suddenly becomes less. It therefore abandons much of its 

load then and there. Repeated deposition in this position, in excess 

of that over other parts of the flood plain, gives rise to the levees. 
Scour-and-fill.| Aggrading streams deepen their channels period- 
1 Hill, Erosion and Deposition by the Indus. Geol. Mag., July, 1910. 


ALLUVIAL DEPOSITS 113 


ically to a notable extent, and the deepening of the channel takes 
place at the very time when the flood-plain is being aggraded. In 
other words, the stream in flood aggrades its plain, and degrades 
its channel. This follows from the fact that the current is slow 
on the plain, where the water is shallow, and rapid in the channel, 
where it is deep. After the flood subsides, the channel, deepened 
while the current was torrential, is filled up again by sediment from 


Pa eee ae ob ¥ 


ase 
“Peep ae 





Fig. 115 Fig. 116 
Fig. 115. Diagram illustrating an early stage in the development of river 
meanders. ‘The dotted area represents the area over which the stream has worked. 
Fig. 116. A later stage in the Cevelopment of meanders. 


the feebler current. This alternate deepening and filling is scour- 
and-fill. It is well illustrated by the Missouri River. At Nebraska 
City, scour reaches depths of 70 to go feet occasionally. At Blair, 
about 25 miles above Omaha, the same river is believed to cut to 
bed-rock (about 40 feet below the bottom of the channel in low 
water) during floods. All streams similarly situated do a like work. 
The material thus eroded is shifted down-stream, some of it for short. 
distances only, and some of it to the sea. An aggrading stream, 
therefore, is not without erosive activity; it is a stream whose fili 
exceeds its scour, not one which has ceased to erode. 

Materials of the flood-plain. Asa result of its varying velocities 
in flood and low water, a stream may deposit coarse material at 


114 WORK OF RUNNING WATER 


one time and fine at another. Many flood-plain deposits are, 
therefore, very heterogeneous, ranging from the finest mud, through 
sand, to gravel, and even bowlders. In general they become finer 
down-stream. 

Flood-plain meanders. A stream with an alluvial plain is 
likely to meander widely (Pls. XI and VII). In general terms 
this may be said to be the result of low velocity, which allows 
the stream to be turned aside easily. Were the course of such a 
stream made straight, it would soon become crooked again. The 
manner of change is illustrated by Figs. 115 and 116. If the 
banks are less resistant at some points than at others, as is always 
the case, the stream will cut in at those points. If the configuration 
of the chaanelat is such as to direct a current against a given point, 

: b (Fig. 115), the result is the 
same, even without inequality 
of material. Once a curve in 
the bank is started, it is in- 
creased by the current which 
is directed into it. Further- 
more, as the current issues from 
the curve, it impinges against 
the opposite bank and develops 
a curve at that point. The 
water issuing from this curve 
develops another, and so on. 

Once started, the curves or 
meanders tend to become more 
and more pronounced (Fig. 
} 116). In the case represented 

by Fig. 1, Pl. VI, the narrow 
neck of land between curves is 
almost cut through. A later 
stage in the process is shown in 
Fig. 2. When the stream has 
cut off a meander, the aban- 
| ag: doned part of the channel may 

Fig. 117. Meanders and cut-offs in the Temain unfilled with sediment. 


Ree ree Valley below Vicksburg. The [f it contains standing water, 
gure shows the migration of the meanders : 

down stream, and their tendency to in- ie sgertety do, it becomes a lake 
crease. (Fig. 117). Some such lakes 










RiverBanks (883 

















Bars etc. 1883 





River Banks 18956 



































ALLUVIAL DEPOSITS 115 


NTCHARTRAIN 


yAKE PO 





Fig. 118. Delta of the Mississippi. The dotted line outside the land represents 
the 3-fathom line. 


have the form of an oxbow, and so are called ox-bow lakes (Fig. 
117, and Pls. VII and XI). 


116 WORK OF RUNNING WATER 


3. Atdebouchures. Where a swift stream flows into sea or 
lake, its current is checked promptly and soon destroyed altogether, 
and its load is dropped. If not washed away by waves, etc., the 
deposits of river-borne sediment in such places make deltas. 

A delta has some features in common with an alluvial fan. In 
both cases the principal deposit is concentrated at the point where 


bi 
gece 





Fig. r19. A delta in a lake. The village is Silva Plana, in the Engadine, 
Switzerland. (Robin.) 


the velocity is checked. In the case of the delta, however, the cur- 
rent is checked more completely, and the debris accumulates (at 
the outset) below the surface of standing water. Though started 
below water, deposition on the surface of a delta may build it up to, 
and even above, the water-level. That part of the delta above 
water is like a flat alluvial fan. 
In profile, the delta differs from 
the alluvial fan in that its edge 
has a steep slope (compare Figs. 
121 and 122). 

Much land has been made by 
delta-building. Thus the Colo- 
rado River has built a great delta 
many square miles (above water) 
in area at the head of the Gulf of 
Ufo) California (Fig. 123). The delta 
yb Sirs} has been built quite across the 





ze . si . "a : * aa a f eS “ ° = A : I 
sists SLE Aes] off its head. In thearid climate of 


Fig. :20. The delta of the Nile. the region, this shut-off head has 





gulf near its upper end, shutting — 


— 


i 


PLATE XI 


Sie 
Wik 





The Aiaocnir: and Big 





bout 2 miles per 


a 
Geol. Surv.) 


"Se 
a 


Sioux Rivers 
Neb. Sheet 





Ta. 





Z 

SS 

MQ 

4S 

qa 

o— 

js) 

A, 

av, | 

i ml 

be a) 

aS) 
~~ 

S . 

ss 

. a 
® 
~ 
~~ 
M 


PLATE XIl 


it 
Y, 
Gf ; 
x 
; { 
A t 
Cs I 
| ard i 
£ bi 
It : 
=" 
; pi 
4 4 
A uf 
. Sa f 
(se Tes 
ss 
f ~ 
wr 


LS j y 
i : f 
3 GLACIERS YA Itt LZII-Zz 
SPEAK) ie . ( 
Wee B | 


Black Mt, 


<i ~—% > Ee NOW <4 SS) 
+ : *& bas * 


Glaciers on Glacier Peak, Washington. Scale, about 2 miles per inch. 
(Glacier Peak Sheet, U. S. Geol. Surv.) 


ALLUVIAL DEPOSITS 117 


become a nearly dry basin, the lowest part of which is about 300 
feet below sea-level. The Skagit River, in Washington, has built 
its delta out so as to surround what were high islands in Puget 
Sound, thus joining them to the mainland. The deltas of the 
Mississippi (Fig. 118), the Nile and the Hoang-Ho Rivers are 











FREE SE. ~ 
eG G AA. LEO OES Te, 
Cg LOLS a 





Fig. 121. Diagrammatic profile and section of a delta. 


well-known. The united delta of the Ganges and Brahmaputra 
is also a great one, having an area (above water) of some 50,000 
square miles. The Po has built a delta 14 miles beyond the former 
port of Adria, which gave its name to the Adriatic Sea. The Rhone 
River (France) has advanced its delta some 15 miles in as many 
centuries. 

The effect of delta-building is to increase the area of the 
land; but it is to be noted that the processes which lead to delta- 
‘building reduce the volume of the land-masses, even though they 
increase their area. 

The outline of some deltas is determined by the surroundings 
in which they are built. When, for example, a delta is built inte 
a bay, the form of the bay-head determines the shape of the delta. 





Fig. 122. Diagrammatic profile and section of an alluvial fan. 


The normal form of a delta built on an open coast is somewhat 
semicircular, though there is in many cases a fringe of delta fin- 
gers which together have some resemblance to the Greek letter A. 
which gave these terminal deposits of streams their names. 


118 WORK OF RUNNING WATER 


ALLUVIAL TERRACES 


Stream terraces ! are bench-like flats or narrow plains along the 
sides of valleys (Fig. 124) and above their bottoms. Most of 
them are narrow, but some of them have great length. 


AN 
ot 


REC AM ATTACH SEAVICE US ae 
“peUikE MAR OR Tae 


“LOWER COLORADO RIVER. 
SROWANG IRRIGABLE LANDS 


sdamente  y TPR ener 


UNITED STATES & MEXICO, 


4 MAAR ROE 





Fig. 123. Relief. -map of an area about the head of the Gulf of California, show- 
ing the delta of the Colorado River, outlined, in a general way, by dotted lines. 
The Salton Sink is shown at the north, and the Imperial Valley lies south of the 
sink. (U.S. Rec. Serv.) 


1 For discussions of terraces see Gilbert’s Henry Mountains, p. 126; Davis, 
Bull. of the Mus. of Comp. Zool. Geol. Series, Vol. V, pp. 282-346; and Dodge, 
Proc. Boston Soc. of Nat. Hist., Vol. XXVI, pp. 257-273. 


ALLUVIAL TERRACES 119 


Most river terraces are remnants of former flood-plains, below 
which the streams which made them have cut their channels, but 
the details of their history are various. 

Normal alluvial terraces. Alluvial terraces are developed in the 
normal course of every stream’s history, because the first graded 
plain which a stream develops in its valley is above the level to which 
the:stream can cut at a later time. After the stream has sunk its 
channel well below the former flood-plain, such parts of the latter 
as still remain are alluvial terraces. Where a stream’s deepened 
channel is in the middle of its flood-plain, there is a terrace on either 





Fig. 124. Terraces on the Fraser River at Lilloet, B.C. (Photo. by Calvin.) 


side; but wherever the deepened channel is at one margin of its 
flood-plain, a terrace remains on the other side only. In some 
valleys there are several alluvial terraces at different levels. The 
second terrace (regarding the highest as the first) is developed in 
the same way as the first, for after the stream has developed a second 
flood-plain, below the level of the first, it may cut its channei still 
lower, leaving the remnants of the second flood-plain as terraces. 
This process may continue until several sets of terraces have 
been developed. Alluvial terraces developed by the normal activi- 
ties of a stream are always low, and ordinarily would not be 
conspicuous. They are not very long-lived, for all processes of sub- 
aérial erosion conspire to destroy them. A stream is likely to mean- 


120 WORK OF RUNNING WATER 


der on its second and later flood-plains, as on its first and highest one. 
Wherever the meanders on its second flood-plain undercut the first 
terrace, the terrace at that point is subject to destruction, and since 
the meanders are continually migrating, terraces are continually 
disappearing. Again, tributary streams cut through the terraces 
of their mains, and new gullies develop in them, dissecting them still 
further. At the same time, sheet erosion and other phases of slope 
wash tend to drive the scarps of the terraces back toward the bluff 
beyond. By the time a second set of terraces is well developed, no 
more than meagre remnants of the first may remain.! 

Other river terraces. There are valley terraces which do not 
represent necessary stages in a valley’s history. (1) Some are due 
to inequalities of hardness (Fig. 80). (2) Again, if an alluvial flood- 
plain has been built as the result of an excessive supply of sedi- 
ment (p. 112), the exhaustion or withdrawal of the excessive sup- 
ply would leave the stream relatively clear, and free to erode where 
it had been depositing. It would forthwith set to work to carry 
away the material which it had temporarily unloaded on the plain. 
The valley plains built up in many valleys in the northern part 
of our continent during the glacial period, when drainage from 
the ice owed through them, have been partially destroyed since 
and their remnants are terraces. (3) A notable increase in the 
volume of a stream, without corresponding increase in load, as 
when one stream captures another, may occasion the develop- 
ment of terraces by allowing the enlarged stream to deepen its chan- 
nel. (4) The uplift of a region in which there are well developed 
river flats, would rejuvenate the streams, and parts of their old 
flood-plains would be left as terraces. Other occasional causes which 
need not be mentioned here, develop terraces from flood plains. 

In conclusion, it is to be emphasized that many river terraces, 
mostly very low, are normal features of valley development, coming 
into existence at definite stages in a valley’s history. They are 
generally composed, in large part, of river alluvium. Others result 
from more or less accidental causes, working singly or in conjunc- 
tion, and to this class belong many of the more conspicuous terraces 
developed from flood-plains. 


Laboratory work. See excercises III-IX, in laboratory manual Interpretation 
of Topographic Maps; also Professional Paper 60. U.S.G.S. Pls. XXIII-LXXXIX. 


1 For a fuller statement of the manner in which alluvial terraces are devel- 
oped, see the authors’ Geologic Processes. 


CHAPTER: V 
THE WORK OF SNOW AND ICE 


Ice beneath the surface. The wedge-work of ice in the crevices 
of rock has already been mentioned (p. 25). When the great 
areas where water freezes during some part of the year are con- 
sidered, it is clear that the aggregate effect of its freezing in the 
pores and crevices of rock must be great in long periods of time. 
Even the freezing of water in the soil is not without effect. This 
is Shown by the disturbance of the walls of buildings if their founda- 
tions are not below the depth of freezing, and by the working up of 
stones and bowlders through the soil of the fields, as freezing and 
thawing succeed each other. Frozen water in the soil makes it 
solid, and temporarily retards or prevents surface erosion. 

Ice on lakes and ponds. Since fresh water is densest at 30° 
Fahr., ice does not commonly form on the surface of a lake until 
the temperature from top to bottom is reduced to this point. .Cooled 
below 39°, the surface water fails to sink, and cooled to 32°, it freezes. 
If the lake is small and shallow, it will freeze over completely where 
the temperature is notably below 32° for any considerable period 
of time. It is under these circumstances that lake ice becomes 
most effective. 

Let us suppose a lake in temperate latitudes, where the range 
of winter temperature is considerable, to be frozen over when the 
temperature is 25° Fahr. If now the temperature is lowered to 
—1o-, and sucha temperature is not uncommon in the northern part 
of the United States, the ice contracts. In contracting, it either 
pulls away from the shores, or cracks. If the former, the water 
from which the ice is withdrawn quickly freezes; if the latter, water 
rises in the cracks and freezes there. In either cases, the ice-cover 
of the lake is again complete. If the temperature now rises to 25° 
the ice expands, and the solid cover becomes too large for the lake, 
and must either crowd up on the shores, or arch up (wrinkle) 
elsewhere. , 

If the water near the shore is very shallow, the ice freezes to the 

121 


122 WORK OF SNOW AND ICE 


sand, gravel, and bowlders at the bottom. If the land at the shore 
is very low, the ice in expanding may shove up over it, carrying the 
debris frozen in its bottom, and it may push loose gravel, sand, etc., 
in front of its edge. Where bowlders are frozen to the bottom of 
the ice, the shoreward thrust as the ice expands shifts them toward 
the shore, and they may be shoved up a little above the normal 
water-level. The concentration of bowlders at the shore-line, year 





Fig. 125. Shore of Wall Lake, Iowa. (Photo. by Calvin.) 


by year, gives rise to the ‘‘walled” lakes (Fig. 125), which are 
not uncommon in the northern part of the United States. The 
‘‘wall’’ does not commonly extend entirely around a lake. 

If a lake is bordered by a low marsh, the ice and frozen earth 
of the latter are really continuous with the ice of the lake, and the 
push of the latter may arch up the former into distinct ridges 
(anticlines), the frozen part only being involved in the folds (Fig. 
126). A succession of colder and less cold periods may give rise 
to a succession of such anticlines.! If the shore is steep, the crowd- 
ing of the ice against a low cliff of yielding material, such as clay, 
disturbs all above the shore-line (Fig. 127). Where the cliff is 
sufficiently resistant, it withstands the push of the ice, and the ice 
itself is warped and broken. 

On rivers. Rivers also freeze over in cold climates, and when 
the ice breaks up in the spring, the stones and bowlders to which it 
was frozen in the banks may be floated miles down the river. At 


1 Buckley, Wis. Acad. of Sci., Vol. XIII, Pt. I, r9g00. A study of ice ramparts 
formed about the shores of Lake Mendota, Wis., in 1898-99. 


RIVER ICE 123 





Fig. 126. Shove of shore ice where the shore was marshy. The ice of the 
frozen marsh is pushed up into ridges. (Buckley, Wis. Geol. Surv.) 


Montreal stone buildings 30 to 50 feet square, projecting so as to 
have river ice form about them, have been moved by the ice of the 
St. Lawrence. 

When the river ice breaks up, masses of it are carried down- 





Fig. 127. The shove of ice on the shore of Lake Mendota, Wis. (Photo. wy 
Buckley.) 


124 WORK OF SNOW AND ICE 


stream, and in some cases accumulate in vast ‘‘jams” behind 
obstructions in the river. Where a jam forms above a bridge, the 
bridge may be swept away. Some jams occasion disastrous floods 
above their sites, and when they break, the waters accumulated 
above may sweep down the valleys with destructive violence. 
Poleward-flowing rivers are especially subject to such floods. The 
snows of their upper basins melt while the lower parts of the streams 
are still frozen over. The free discharge of the upper waters is 
thus prevented for a time, and freshets follow. 

On the sea. In high latitudes, ice is formed along the sea- 
shore. Unlike fresh water, sea-water condenses until it freezes, 
at a temperature of 26° to 28° Fahr., the variation being due to 
the amount of salt in the water. In polar regions the sea ice attains 
a depth of eight or ten feet at least. Floating ice of much greater 
thickness is sometimes seen, but it is doubtful if it represents ice 
formed by the freezing of undisturbed sea-water. The geologic 
importance of ice formed on the sea is slight. Lee 

Snow-fields. Over the larger part of the land, the snow of winter 
does not endure through the summer, and when it melts, the water 
follows the same course as rain; but in cold regions where the fall 
of snow is heavy, some of it remains unmelted from year to year, 
and constitutes perennial snow-fields. High mountains and the 
lands of high latitudes are the common habitats of snow-fields. In 
North America there are numerous small snow-fields in the western 
mountains, from Mexico to Alaska, their number and size increas- 
ing to the north. In the United States there are few snow-fields 
south of the parallel of 36° 30’, and most of the many hundreds 
north of that latitude (excluding Alaska) are small. Snow-fields 
comparable to those of the northwestern part of the United States 
and British Columbia occur in the higher mountains of Europe and 
Asia, while in South America there are snow-fields of small size even 
in equatorial latitudes. Small snow-fields occur on the highest 
peaks of tropical Africa, and in the mountains of New Zealand. 
For reasons which will APH SAS later, much of every large snow-field 
is really ice. 

Besides these fields Re snow in mountain regions, there are idlds 
of much greater extent in polar regions. The greater part of Green- 
land is covered with a single field of ice and snow, the size of which 
is estimated at 300,000 to 400,000 square miles (Fig. 128),—an area 
400 to 600 times as large as the snow-and-ice-covered area of Swit- 


SNOW TO ICE 125 


zerland. Numerous islands to the west of North Greenland are also 
partly covered with snow. In Antarctica there is a still larger 
field, the largest of the earth. Its area is not known, but its ex- 
tent is at least 6 or 8 times as great as that of Greenland. 

The only condition necessary for 
a snow-field is an excess of snow-fall 
over snow-waste. The lower edge of 
a snow-field, the snow-line, is de- 
pendent chiefly on temperature and 
snow-fall. It does not depart much 
from the summer isotherm of 32°, 
though where the snow-fall is light, 
it may be above this isotherm. That 
the snew-line is not a function of 
temperature only, is shown by its 
position in various places. Thus in 
the equatorial portion of the Andes, 
the snow-line has an altitude of about 
16,000 feet on the east side of the 
mountains, where the precipitation 
is heavier, and of about 18,500 feet 
on the west side, where it is lighter. 
For the same reason the snow-line 
in the Himalayas is lower on the 
south side than on the north. Though ees 





Map showing the 
temperature and snow-fall are the  icé-cap of Greenland. Only the 


most important factors controlling borders (shaded parts) of the island 


the position of the snow-line, both “~ peer Ra ae 


humidity and movements of air are of some importance, since both 
affect the rate of evaporation of snow and ice. 

Change of snow to ice. Snow does not lie on the surface long 
before it undergoes obvious change. The light flakes are trans- 
formed into granules, and the snow becomes ‘‘coarse-grained.”’ 
The granular character, so pronounced in the last banks of snow in 
the spring, is even more distinct in perennial snow-fields. This 
granular snow is called névé. Where the thickness of the snow is 
great, the névé becomes compact below, and grades into porous ice. 
Ice is found in some snow-fields at no great depth from the surface. 

Structure of the ice. The ice of a snow-field is in some sense 
stratified. It is made up of successive falls of snow which tend to 


126 WORK OF SNOW AND ICE 


retain their individuality. Thus the snow of one season may have 
been considerably changed before the next season. Again, the sur- 
face of the snow-field at the end of the melting season is generally 
soiled by a little earthy matter, some of which was blown up on the 
surface during the melting season, and some of which was concen- 
trated at the surface by the melting of the snow in which it was 
originally imbedded. In many places this earthy matter is sufficient 
to ‘define snows of successive years, giving the ice a somewhat 
stratified appearance. 

In addition to its stratification, the ice of the deeper portions 
may take on a stratiform structure which may be called foliation, 
to distinguish it from the stratification which arises from deposition. 
Foliation appears to be akin to slaty or schistose cleavage, and to 
result largely from the shearing of one part of ice over another, as 
it moves forward. , 

Texture. Ice formed from snow is composed of interlocking 
crystals. The crystalline character is assumed by the snow-flakes 
when they form, and the subsequent changes which the snow under- 
goes seem to modify the original crystals by building up some and 
destroying others. By the time the snow is converted into névé, 
the granules have become coarse, and wherever the ice derived from 
the névé has been examined, the granular crystalline texture is 
present. The individual crystals in the ice are usually larger than 
those of the névé, and more closely grown together. In compact 
ice, the crystals are so intimately interlocked that they are not 
seen readily by the eye; but when the ice has been honeycombed 
by partial melting, the granules become partially separated and may 
be seen easily. It is therefore legitimate to assume that a granu- 
lar crystalline condition persists throughout all stages of the history 
of ice formed from snow. 

Inauguration of movement. When the ice beneath a snow-field 
becomes very deep, motion is developed. The exact nature of the 
motion has not been demonstrated to the satisfaction of all who have 
studied the problem, though much is known about it. Brittle and 
resistant as ice seems, it may, under proper conditions, be made 
to exhibit some of the characteristics of a plastic substance. A 
piece of ice may be made to change its form, and may even be mould- 
ed into almost any desired shape if subjected to sufficient pressure, 
applied steadily through long intervals of time.' These changes 
may be brought about without visible fracture, and have been 


GLACIERS 127 


thought to point to a viscous condition of the ice. There is much 
reason, however, to question this interpretation. Whatever the 
real nature of the movement, its aggregate result in a field of ice is 
‘comparable, in a superficial way at least, to that which would occur 
if the ice were capable of moving like a viscous liquid, the motion 
taking place with extreme slowness. This slow motion of ice in 
an ice-field is glacier motion, and ice thus moving is glacier ice. 
The cause of movement is gravity, which tends to bring the ice to 
lower levels, just as it tends to bring water, in similar positions, to 
lower levels. 
GLACIERS 

Types. The different shapes of glaciers have given rise to differ- 
ent names. If the surface on which the ice-sheet develops is plane, 
the ice will move outward in all directions, and ice spreading in 
all directions from a center is an ice-cap. The glacier covering the 
larger part of Greenland (Fig. 128) is a good example. The glaciers 
on some of the  flat-topped 
peninsular promontories of the 
same island are examples of 
small ice-caps (Fig. 129). If 
ice-caps cover a large part of a 
continent, as some of those of 
the past have done, they are 
called continental glaciers. 

Where ice-caps lie on pla- 
teaus whose borders are dis- 
sected by valleys, tongues of 
ice from the ice-cap may ex- 
tend down the valleys. They Fig. 129. Ice-caps of small size. The 


. figure also shows some valley glaciers 
constitute one type of valley extending out from the main ice-sheet 


glacter. A second and more and from the local ice-caps. A portion 
familiar type of valley glacier oe Ean cms Spree aan of 
occupies mountain valleys, and 

is the offspring of mountain snow-fields. The former type, confined 
chiefly to high latitudes, are polar or high-latitude glaciers (Fig. 130); 
the latter are alpine glaciers (Figs. 131, 132). The distinctive feat- 
ure of high-latitude glaciers is their steep slopes at sides and ends. 





1 For an account of experiments illustrating the mobility of ice see Aitkin, 
Am. Jour. Sci., Vols. V, 1873, Pp. 395; and XXXIV, 1887, p. 149, and Nature, Vol. 
XXXIX, p. 203. 


128 WORK OF SNOW AND ICE 





Fig. 130. End of Bryant glacier, a high-latitude glacier of North Greenland. 











Fig. 131. The Rhone glacier. (Photo. by Reid.) 


GLACIERS _ TG 





Fig. 132. The medial moraine of the Roseg Glacier, Switzerland. 


» .When a valley glacier descends through its valley to a plain 
beyond, its end spreads. | If the deploying ends of adjacent glaciers 
merge, the resulting body of ice constitutes a predmont glacier (Fig. 
133). Piedmont glaciers are confined to high latitudes. In some 









sy \y \ + 
N XY Ni 
y 


o> 













ish 


Py Yalrutat Bay 














Pactlic Ocear fla val 


Fig. 133. Malaspina Glacier, a piedmont glacier in Alaska. (After Russell.) 


130 WORK OF SNOW AND ICE 


cases the snow-field that gives rise to a glacier is restricted to a 
relatively small depression in the side of a mountain, or in the 
escarpment of a plateau. In such cases the snow-field and glacier 
are hardly distinguishable, and the latter descends but little below 
the snow-line. Such a glacier, nestled in the face of a cliff, has been 
called a cliff glacier! (Fig. 134). Cliff glaciers may be as wide as 





Fig. 134. A cliff glacier, coast of North Greenland. The height of the cliff 
is perhaps 2,000 feet. The water in the foreground is the sea. 


long, and are always small. Between them and valley glaciers 
there are all gradations (Fig. 135). Occasionally the end of a valley 
glacier, or the edge of an ice-sheet, reaches a precipitous cliff, and the 
end or edge of the ice breaks off and accumulates like talus below. 
The fragments of ice may then become a coherent mass by regela- 
tion, and the whole may resume motion. Such a glacier is called a 
reconstructed glacier. The precipitous cliffs of the Greenland coast 
furnish illustrations. 

Of the foregoing types of glaciers, ice-caps far exceed all others 
in both size and importance, while valley glaciers outrank the 
remaining types; but since valley glaciers are the most familiar, 
the general phenomena of glaciers will be discussed with primary 
reference to them. 

1 Jour. of Geol., Vol. III, p. 888. 


GLACIERS “131 





Fig. 135. Glaciers intermediate in type between cliff glaciers and valley 
glaciers. Cascade Mountains, Wash. (Willis, U. S. Geol. Surv.) 


General Phenomena of Glaciers ' 


Dimensions. Some valley glaciers occupy only the upper parts 
of mountain valleys, others extend through them, and push out on 
the plain beyond. In length they range from a fraction of a mile 
to many miles. Their thickness is usually measured by scores or 
hundreds of feet rather than by denominations of a larger order, 
but the variation is great. The minimum thickness is that which is 
necessary to cause movement, and this varies with the slope, the 
temperature, and other conditions. There is also much variation 
in the thickness in different parts of the same glacier. Asa rule, it 
is thinnest in its terminal portion, and thickest at some point be- 
tween its terminus and its source. Cliff and reconstructed glaciers 


1 The following list includes some of the more available articles and treatises 
on existing glaciers; others are referred to in the following pages. 

Alaskan glaciers: Reid, (1) Nat. Geog. Mag., Vol. IV, pp. 19-55; (2) Sixteenth 
Ann. Rept., U. S. Geol. Surv., Part I, pp. 421-461. Russell, (1) Nat. Geog. Mag., 
Vol. III, pp. 176-188; (2) Jour. of Geol., Vol. I, pp. 219-245. 

Glaciers in the United States: (1) Russell, Eighteenth Ann. Rept., U. S. Geol. 
Surv., Part II, pp. 379-409; (2) Glaciers of North America. 

Greenland glaciers: Chamberlin, Jour. of Geol., Vol. II, pp. 768-788; Vol. 
III, pp. 61-69, 198-218, 469-480, 565-582, 668-681, and 833-843; Vol. IV, pp. 582- 
592. Salisbury, Jour. of Geol., Vol. IV, pp. 769-810. 


132 WORK OF SNOW AND ICE 


are comparable in size to the smaller valley glaciers. An ice-cap 
is thickest, theoretically, at its center, and thins away to its borders; 
but its actual thickness is influenced by the topography of the sur- 
face beneath it. The Greenland ice-cap rises about 9,000 feet above 
the sea toward its southern end, and it probably rises higher in the 
unexplored center of the broader part of the island. The height of 
the rock surface beneath the ice is unknown, but it is unlikely that: 
it averages half this amount, and hence the ice is probably very 
thick at its center. 

Limits. The ice of a glacier is always moving forward, but the 
end of a glacier may be retreating, advancing, or remaining station- 
ary, according as waste exceeds, falls short of, or equals forward 
movement. The position of the lower end of a glacier is therefore 
determined by the ratio of movement to waste. Its upper end is 
generally ill-defined. In a superficial sense, it is where the ice 
emerges from the snow-field; but the lower limit of the snow-field 
is ill-defined, and in any case is not the true upper limit of the 
glacier. The snow-field is really an ice-field covered with snow, 
and there is movement from it to the tongue of ice in the valley. 
The ice so moving is, in reality, a part of the glacier. The lower end 
of a glacier is usually free from snow and névé in summer, but its 
upper end is covered with névé or snow, and finally merges into the 
snow-field without ceasing to be a glacier. The term glacier is, 
however, commonly used to mean merely the more solid portion 
outside (below) the snow-field. 

Movement. The advance of a glacier is too slow, as a rule, to 
be seen from day to day, but is detected in other ways. If its end 
advances, it overrides or overturns objects which were in front of 
it, or it moves out over ground previously unoccupied. But even 
when the end of a glacier is not advancing, movement of the ice 
may be established by means of stakes or other marks on its surface. 
If the position of these marks relative to fixed points on the sides 
of the valley is noted, they are found, after a time, to have moved 
down the valley. | 

Rows of stakes or lines of stones set across a glacier in its upper, 
middle and lower portions have revealed many facts concerning the 
movement of the ice. Generally speaking, the central part moves 
‘faster than the sides, and the top faster than the bottom. In 
Switzerland the determined rates of movement range from one or 
two inches to four feet or more per day. Some of the larger glaciers 


MOVEMENT OF GLACIERS 133 


in other regions move more rapidly, but it does not follow that 
large glaciers always move faster than small ones. The Muir 
glacier of Alaska has been found to move some seven feet per day,} 
and some of the glaciers of Greenland move, in the summer time, 
so or 60 feet per day; but these rates have been observed only where 
the ice of a large inland area crowds down into a comparatively 
narrow fiord, and debouches into the sea, and there only in the 
summer. In the case of the glacier with the highest recorded 
summer rate of movement (1oo feet per day), the advance was 
only 34 feet a day in April. The average movement of the border 
of the inland ice of Greenland is very small, probably less than a 
foot a week. 

Conditions affecting rate of movement. The rate of glacier 
movement depends on (1) the depth of the moving ice, (2) the slope 
of the surface over which it moves, (3) the slope of the upper surface 
of the ice, (4) the topography of its bed, (5) the temperature of the 
ice, and (6) the amount of waterit contains. Great thickness, steep 
slopes, smoothness of bed, a high (for ice) temperature, and abund- 
ance of water, favor rapid movement. Since some of these condi- 
tions, notably temperature and amount of water, vary with the 
season, the rate of movement of a glacier varies during the year. 
Other conditions vary through longer periods of time, and cause 
corresponding variations in the rate of movement. 

A sloping upper surface is essential to glacier motion, and the 
motion is down-slope. ‘There are short stretches where this is not 
the case; indeed there are places where the upper surface declines 
away from the direction of motion, as where the ice pushes up over 
a swell in its bed; but such cases are local exceptions and do not 
militate against the general truth of the statement that the upper 
surface of a glacier declines in the direction of motion. A declining 
lower surface is less necessary. In the case of a valley glacier, the 
bed does, as a rule, decline in the direction of motion; but the deep 
basins in rock which many such glaciers leave behind them when 
they retreat, show that the bottom of a valley glacier does not slope 
downward at all points. In the great continental glaciers of recent 
geologic times, the ice moved up slopes for scores, and even hundreds 
of miles; but in all such cases, the prevailing slope of the upper sur- 
face was down in the direction of movement. 

Fluctuations of glaciers. The lower ends of glaciers advance 

1Reid. Natl. Geog. Mag., Vol. IV, p. 44. 


134 WORK OF SNOW AND ICE 


and retreat at intervals', and the periods of advance follow a suc- 
cession of years when the snowfall was heavy and the temperature 
low, while the periods of retreat follow years when the snowfall 
was light and the temperature 
above normal. The periods of 
advance and retreat lag behind 
the periods of heavy and light 
snowfall, respectively, by some 
years, and a long glacier re- 
sponds less promptly than a 
short one. 

Likenesses and _ unlike- 
nesses of glaciers and rivers. 
Slope, roughness of bed, and 
volume affect the movement of 
glaciers somewhat as they af- 
fect the movement of rivers. 
The temperature of water, on 
the other hand, has little effect 
on its flow, so long as it remains 
unfrozen; but the effect of 
temperature on the motion of 
ice is important. In many 
cases, indeed, the tempera- 
ature, together with the water 
that is incidental to it, seems to 
be the chief factor in determin- 
ing its rate of movement. Its 
effects will be discussed later. 

From Fig. 136 it will be seen that a valley glacier is an elongate 
body of ice, following the curves of the valley in stream-like fashion. 
It has its origin in the snows collected on the mountain heights, and 
it works its way down the valley in a manner which, in the aggre- 
gate, is similar to the movement of a stiff liquid. The likeness 
to a river extends to many details. Not only does the center 
move faster than the sides, and the upper part faster than the 
bottom, as in the case of streams, but the movement is more rapid 
in the narrow parts of the valley and slower in the broader. These 





Fig. 136. Aletsch Glacier, Switzerland. 


1 Reid. Variations of Glaciers. Occasional articles in Journal of Geology, 
Vo}. III and later volumes. 


ee 


MOVEMENT OF GLACIERS 136 


and other likenesses, some of which are apparent rather than real, 
gave rise to the view that glacier ice moves like a stiff, viscous 
liquid. 

But while the points of likeness between glaciers and rivers are 
several, their differences are numerous and significant. The most 
obvious difference is the fact that the glacier is fractured readily, 
as the numerous gaping crevasses on many glaciers show. Some 
of the crevasses are longitudinal, some are transverse, and some 
are oblique. In the case of arctic glaciers, longitudinal crev- 





Fig. 137. Crevassed glacier, the cracking due to change in grade of bed. 
North Greenland. 


assing is especially conspicuous. Crvevasses appear to be de- 
veloped wherever there is appreciable tension, and the causes 
of tension are many. An obvious cause is an abrupt increase of 
gradient in the bed (Fig. 137). If the change of gradient is con- 
siderable, an ice-fall or cascade results, and the ice may be greatly 
riven. Some of the transverse crevasses at the margins appear 
to be the result of tension developed on curves. Oblique crevasses 
on the surface near the sides are commonly ascribed to the tension 
between the faster-moving center and the slower-moving margins, 
and in like manner cracks that rise obliquely from the bottom are 
attributed to the tension between the faster-moving parts above 
and the slower-moving parts below. All crevasses indicate strains. 
Liquids, whose pressures are equal in all directions, show nothing 
analogous to crevassing. Longitudinal crevasses may affect both 
the narrow part of a glacier and its deploying end, and are the result 
of tension developed by movement within the ice itself, to which, 


136 WORK OF SNOW AND ICE 


again, rivers offer no analogy. All cracks show that the glacier is 
a very brittle body, incapable of resisting even very moderate strains 
brought to bear upon it very slowly. In its behavior under tension, 
therefore, a glacier is notably unlike a river. 

Surface moraines. ‘The surfaces of many glaciers are affected 
by rock debris, some of which is disposed in the form of belts or 
moraines (Fig. 138). The 
surface moraines may be 
lateral, medial, or ter- 
minal. A lateral moraine 
is any considerable accu- 
mulation of debris in a 
belt on the side of a gla- 
cier. A medial moraine 
is a similar accumula- 
tion at some distance 
from the margins, but 
not necessarily in or even 
near the middle. There 
may be several medial 
moraines on one glacier. 
In valley glaciers, the 
surface terminal moraine 
may connect two lateral 
moraines, making a loop 
roughly concentric with 
the end of the glacier. 
Besides the surface mo- 
raines, there may be 

Fig. 138. Lateral and medial moraines, scattered bowlders and 
the latter formed by the union of glaciers. ; > 
bits of rock of various 
sizes on the ice, and, in addition to the coarse material, there is in 
many cases some dust which has been blown upon the ice. 

Relief due to surface debris. The debris on the ice affects its 
topography by influencing the melting of the ice beneath and about 
it. Rock debris absorbs heat more readily than the ice. A thin 
piece of stone lying on the ice is warmed through by the sun’s rays, 
and, melting the ice beneath, sinks, just as a piece of black cloth 
would. Though a good absorber of heat, rock is a poor conductor, 
and so the lower surface of a thick mass of stone is not warmed 





= an 


RELIEF OF GLACIERS 137 


notably, and the ice beneath, being protected from the sun, is 
melted less rapidly than that around it. The result is that the 
bowlder presently stands on a protuberance of ice (Fig. 139). When 
its pedestal becomes high, the oblique rays of the sun and the 
warm air surrounding 
it cause it to waste 
away, and the cap- 
ping bowlder falls. 

The same _ prin- 
ciples apply to mo- 
raines. A_ surface 
moraine protects the 
ice beneath from 
melting, and causes 
the development of 
a ridge of ice beneath 
itself. As the ice on 
either side is lowered 
by ablation, the mo- 
raine matter tends to 
slide down on either 
hand. So far does this 
spreading go, that in 
some cases the lower 
end of a glacier is 
completely covered 
with debris which 
has spread from me- 


dial and lateral mo- ~- Fig. 1309. A glacial table due to the protection of 
raines. the ice beneath the flat stone from the rays of ‘the sun. 


enceinslow the Taléfre Glacier. 
surface. Debris carried by a glacier is not restricted to its upper 
surface. Debris near the bottom is in some cases so abundant, espe- 
cially near the ends and edges of the ice, that it is difficult to locate 
the bottom of the glacier; for between the moving ice which is full 
of debris, and the stationary debris which is full of ice, there seems 
to be complete gradation. The debris in the lower part of arctic 
glaciers, and to some extent of others, is in many cases disposed in 
thin sheets between layers of clean ice. Debris also occurs to some 
extent in the ice far above its base, in some places in sheets and in 





138 WORK OF SNOW AND ICE 


some places in bunches. These various relations are illustrated by 
Figs. 140 and 141. 

Drainage. Some of the water produced by surface melting 
forms little streams on the ice. Sooner or later they plunge into 
crevasses or over the sides and ends of the glacier. In the former 
case, they may melt or wear out well-like passages (moulins) in the 





Fig. 140. Side view of end of glacier. Southeast side of McCormick Bay, 
North Greenland. Shows foliated structure of ice as well as position of debris. 


ice, and hcles or ‘‘ wells” in the rock beneath. Much of the surface 
water sinks into the ice without forming streams. The depth to 
which: water penetrates is undetermined by observation, but it 
doubtless goes down to the zone of constant temperature in all cases, 
and still lower where there are crevasses, and where the temperature 
is not below freezing. 

Once within the glacier, the course of the water is variable. 
Exceptionally it follows definite englacial channels, as shown by 
the springs and streams which issue from some glaciers above their 
bottoms. More commonly it descends or moves forward through 
the irregular openings which the accidents of motion have made. 
If it reaches a level where the temperature is below 32° it freezes. 
Otherwise it remains in cavities or descends to the bottom. The 


STRUCTURE OF GLACIER ICE 139 


water produced by melting within the glacier probably follows a 
similar course. So far as these waters descend to the bottom, they 
join those produced by basal melting, and issue from the glacier with 
them. In some alpine glaciers, the 
waters beneath the ice unite in 
a common stream in the axis of the 
valley, and hollow out a_ tunnel 
in the bottom of the ice. The 
Rhone River is already a consider- 
able stream where it issues from 
beneath the glacier. In high lati- 
tudes, subglacial tunnels are not 
common, and the drainage is in 
streams along the sides of the gla- 
ciers, or through the debris beneath 
and about them. 

At the end of the glacier, all 
waters, whether they have been 
superglacial, englacial, or subglacial, 
unite to bear away the silt, sand, 
gravel, and even small bowlders set 
free from the ice, and to spread them 
in belts along the border of the ice, 
or in trains stretching down the val- 
leys below, forming glacto-fluvial 
deposits. 





: Fig. 141. A part of the vertical 
The structure and the motion of — gide of a North Greenland glacier. 


glacier ice have been the subject of |The vertical or even overhanging 


faces are in some cases more than 


much discussion. Though univer- 
100 feet high. 


sal agreement concerning them has 

not been reached, a brief outline of one of the current views is added. 
Mention also is made of other views, some of which are still held 
by various geologists. 


THE STRUCTURE OF GLACIER ICE 


The key to the structure and motion of glacier ice is based on the view that a 
glacier is a mass of crystalline rock of the purest and simplest type known. It is 
made of a single simple mineral, ice, which is always crystalline. It differs from 
other rock chiefly in that its one mineral is liquefied at a low temperature. 

The development of ice from snow. The fundamental conception of a 
glacier is best developed by tracing the growth of its constituent crystals. When 


140 WORK OF SNOW AND ICE 


water solidifies from the vapor of the atmosphere, it takes the form of separate 
crystals (Fig. 142). The flakes are rarely perfect, but they are always crystals. 
Snow crystals may continue to grow so long as they are in the atmosphere; or if 
the air is warm or dry they shrink, from melting or evaporation. When they reach 
the ground, the processes of growth and shrinking continue, and the crystals 
increase or decrease according to circumstances. 

A glacier is a colossal aggregation of crystals grown from snowflakes to granules 
of greater size and more compact form. ‘The microscopic study of snowflakes shows 
how they change from flakes to granules. The slender points and angles of new- 
fallen flakes melt and evaporate more than the central portions. The water’ 
(and doubtless the water vapor) thus formed gathers about the centers of the flakes 
and, if the temperature is right, freezes there. 

These are first steps toward the pronounced granulation of snow which has 
lain long on the ground. Measured from day to day, the larger granules beneath 
the surface of coarse-grained snow are found to be growing. When the temper- 
ature of the atmosphere is above the melting-point, the growth is faster than when 
the air is colder, but there is an increase in the average size of the granules, and a 
decrease in their number, under all conditions of temperature. Part of the increase 
of the larger granules appears to be at the expense of the smaller ones; part doubt- 
less comes from the moisture of the atmosphere which penetrates the snow and 
condenses there, and part from the descent of water due to surface melting. 

Deep beneath the surface of a large body of snow, the larger part of the growth 
of the large granules is probably at the expense of the small ones. To understand 
how this takes place, it should be noted that the free surface of every granule is 
constantly throwing off particles of water-vapor (i. e., evaporating); that the rate 
of evaporation increases with the sharpness of the curve of the surface, and that 
the smaller the particles, the sharper the curve; that the surface of a granule is 
liable to receive and retain molecules evaporated from other granules, and that, 
other things being equal, the retention of particles is most common on surfaces 
of least curvature. It follows that the larger granules of less curvature will lose 
less and gain more, on the average, than the smaller granules. The result is that 
the larger granules grow at the expense of the smaller. 

Another factor affecting the growth of granules is pressure and tension. The 
granules are compressed at their points of contact, and under tension elsewhere. 
Tension increases the tendency to evaporation, and the capillary spaces adjacent 
to the points of contact probably favor condensation. Pressure reduces the 
melting-point, while tension raises it. Though the effect of this is slight, it is to 
be correlated with the much more important fact that compression produces heat ~ 
which may bring the temperature of the ice to the melting temperature at some 
points, while tension may reduce it to or below freezing temperature at adjacent 
points. There is therefore a tendency for the ice to melt at points of contact and 
compression, and for the water so produced to refreeze at adjacent points where the 
surfaces of granules are under tension. This process becomes effective beneath a 
considerable body of snow, and here the granules gradually lose the spheroidal 
form assumed in the early stages of granulation, and become irregular polyhedrons, 
interlocked into a mass of more or less solid ice. 

Whether these processes furnish an adequate explanation of the changes or 
not, all gradations may be observed from snowflakes to granular névé, and thence 
to the granules of glacier ice, ranging in size up to that of walnuts, and even 


141 


SNOWFLAKES 





Photographs of snowflakes, enlarged. (Bentley.) 


Fig. 142. 


142 WORK OF SNOW AND ICE 


beyond. In coherence, these aggregations vary from the névé stage, where the 
grains are small and spheroidal, to the ice stage, where the cohesion is strong 
through the interlocking growths of the large granules. 


MOTION OF GLACIER ICE! 


Rotation and sliding of granules. There seems to be no escape from the con- 
clusion that the primal cause of glacier motion is one which may operate even 
under the relatively low temperatures, the relatively dry conditions, and the 
relatively granular textures at the heads of glaciers. These considerations lead to 
the view that movement there takes place by the movements of the grains upon 
one another. While they are in the spheroidal form, as in the névé, this would 
not seem to be difficult. They may rotate and slide over each other as the weight 
of the névé increases, and the motion between the granules might be comparable 
to that between shot in great quantities in similar positions. The amount of 
motion required of an individual granule is surprisingly small. In order to account 
for a movement of three feet per day in a glacier six miles long, the mean motion 
of the average granule relative to its neighbor would be roundly, todo of its own 
diameter per day; in other words, it should change its relations to its neighbors to 
the extent of its diameter once in about thirty years. A change of such slowness 
under the conditions of granular alteration can scarcely be thought improbable. 

Melting and freezing. After the granules become interlocked, as in the body 
of the glacier below the névé field, rotation and sliding must be more difficult. 
Then, if not earlier, the movement between granules is supposed to be effected 
chiefly by the temporary passage of minute portions of the granules into the fluid 
form at points of greatest compression, the transfer of the water thus produced to 
adjoining points, and its resolidification. The points of greatest compression are 
obviously those whose yielding most promotes motion, and the successive yielding 
of points which come in succession to oppose motion most (and thus to receive the 
greatest stresses), permits continuous motion. It is only necessary to assume that 
the gravity of the accumulated mass is sufficient to produce a little temporary 
iiquefaction at the points of greatest stress, the result being accomplished not so 
much by the lowering of the melting-point as by the development of heat by 
pressure. This is believed to be the largest single element in glacier motion. 

This conception of glacial movement involves the momentary liquefaction of 
minute portions of the ice, while the mass as a whole remains rigid, as its crystalline 
nature requires. Instead of assigning a slow viscous fluidity like that of asphalt 
to the whole mags, which seems inconsistent with its crystalline character, it assigns 
a free fluidity, momentarily, to a succession of particles that form only a minute 
fraction of the whole at any instant. This conception is consistent with the reten- 
tion of the granular condition of the ice, with its rigidity and brittleness, and with © 
its strictly crystalline character, a character which a viscous liquid does not possess, 
however much its high viscosity may make it resemble a rigid body. 

Accumulated motion in terminal part of glacier. However slight the relative 
motion of one granule on its neighbor, the granules in any part of a glacier partake 
in the accumulated motion of all parts nearer the source, and hence all except those 
at the head are thrust forward. Herein appears to lie the distinctive nature of 
glacial movement. Each part of a stream of water feels (1) the hydrostatic pres- 


- £or fuller discussion, see the authors’ Geologic Processes, pp. 308-321. 





MOTIONS OF GLACIER ICE 143 


sure of neighboring parts (theoretically equal in all directions), and (2) the mo- 
mentum of motion, but vot the thrust of the water up stream. This is probably one 
of the fundamental differences between water flow and glacier motion. 

Lava streams are good examples of viscous fluids flowing in masses comparable 
to those of glaciers, on similar slopes, and, in the last stages of motion, at similar 
rates; but their special modes of flow and their effects on the sides and bottoms of 
their paths are radically different from those of glaciers. Forceful abrasion, and 
particularly the rigid holding of imbedded stones which score and groove the rock 
beneath, is unknown in lava streams, and is scarcely conceivable. There is, so 


sate Sees e er 


i mas “ 





Fig. 143. A well defined shearing plane in a Spitzbergen glacier. (Hamberg.) 


far as we know, no experimental or natural evidence that any viscous fluid, in the 
ordinary sense of that term, detaches and picks up fragments and holds them firmly 
as graving tools in its base so as to cut deep, long, straight grooves in the hard 
bottom over which it flows. It would seem that competency on the part of a 
“viscous body to do this peculiar class of work so distinctive of glaciers should be 
demonstrated before the viscous theory of glacial movement is accepted as even a 
good working hypothesis. In contrast with viscous movement, it is conceived that 
a glacier is thrust forward rigidly by internal elongation, and that it is sheared 
forcibly over its sides and bottoms, leaving its distinctive marks upon them. 
Shearing. In the terminal part of a glacier, where the thrusts are greatest, 
where the granules are fewest and their interlocking most intimate, shearing takes 
place within the ice. This is illustrated by Figs. 143 and 144. The shearing re- 
sults in the foliation of the ice, and in the dragging of debris along the planes of 
shear. Shearing is obs:rved chiefly where the ice below the plane of shearing is 
protected more or less from the force of the thrust, as in the lee of a hill or mass of 


144 WORK OF SNOW AND ICE 





Fig. 144. Portion of the east face of Bowdoin Glacier, North Greenland, show- 
ing oblique upward thrust, with shear. 


débris. It perhaps occurs also at the top of the basal zone of ice so loaded with 
débris that it is incapable of ready movement. 

It is probable, also, that sharp differential strains and shearing are developed 
at the level where the surface water of the warm season, sinking into the ice, reaches 
the zone of freezing; for the expansion which attends the freezing may cause the 
expanding layer to shear over the part below. As the level of freezing descends 
with the advance of the warm season, the zone of shearing sinks. 

Expansion at the zone where descending water freezes not only leads to shear, 
but to the development of surface cracks, for the surface is stretched as the zone 
below expands. In the course of years, the cracks developed in this way may 
become wide crevasses, limited below by the depth of the zone of freezing in 
summer. | 

High temperature and water. Toward the:lower end of a glacier, the higher 
temperature and the greater abundance of water lend their aid to the fundamental — 
elements of movement. During the warm season, the ice here is bathed in water — 
all the time, so that the necessary changes in the crystals are facilitated. Under 
these conditions, movement takes place more readily than in the drier, colder and ~ 
more open, granular ice of lower temperature, near the source of the glacier. 

Application. The co-operation of these several factors appears to explain the ~ 
peculiarities of glacial movement. In regions of intense cold, where a dry state 
and low temperature prevail, as in the heart of Greenland, the snow-ice mass may 


Pe 







1 The crystals of ice have a peculiar structure which has been thought by some 
to be an important factor in shearing, and so in the motion of glaciers ice. See 
the authors’ Geologic Processes, p. 312; also (10) p. 323. 


MOTIONS OF GLACIER ICE 145 


attain extraordinary thickness. Here the burden of movement seems to be thrown 
almost wholly on compression, with the slight aid of molecular changes due to 
internal evaporation. Since the temperature in the upper part of the ice is very 
low most of each year, the compression must be great before it becomes effective 
in melting; hence the great thickness of ice necessary before motion is considerable. 
Similar conditions affect the heads of Alpine glaciers, though here the high gradients 
favor motion among the granules of ice. In the lower reaches of Alpine glaciers, 
where the temperatures are near the melting-point, and where the ice is bathed in 
water much of the time, movement may take place in ice which is thin and compact. 

If the views here presented are correct, there is also, at all points below the 
source, the co-operation of rigid thrust from behind, with the tendency of the mass 
to move on its own account. The latter is controlled by gravity, and conforms 
in tts results to laws of liquid flow. The former is a mechanical thrust. This 
thrust is different from the pressure of the upper part of a liquid stream on the 
lower part, because it is transmitted through a body whose rigidity is effective, 
while the latter is transmitted on the hydrostatic principle of equal pressure in all 
directions. Thrust would be most effective toward the end or edge of a glacier. 

Corroborative phenomena. ‘The conception of the glacier and its movement 
here presented explains some of the anomalies that otherwise seem paradoxical. 
If the ice is always a rigid body which yields only as its interlocking granules 
change their form by loss and gain, a rigid hold on the imbedded rock at some 
times, and a yielding hold at others, is intelligible. Stones in the base of a glacier 
may be held with great rigidity when the ice is dry, scoring the bottom with much 
force, while they may be rotated with relative ease when the ice is wet. In short, 
the relation of the ice to the bowlders in its bottom varies radically according to its 
dryness and temperature. A dry glacier is a rigid glacier. A dry glacier is neces- 
sarily cold, and a cold glacier 1s necessarily dry. 

It is difficult to explain the furrows and grooves cut by glaciers in firm rock if 
the ice is so yielding as to flow under its own weight on a surface which is almost 
flat. If the mass is really viscous, its hold on its imbedded debris should also be 
viscous, and a bowlder in the bottom should be rotated in the yielding mass when 
its lower point catches on the rock beneath, instead of being held firmly while a 
groove iscut. ‘This is especially to the point since viscous fluids flow by a partially 
rotary movement. 

On the view here presented, a glacier should be more rigid in winter than in 
summer. ‘The total thickness of a glacier should experience this rigidity of winter 
at its ends and edges, where the ice is thin enough to permit the low temperature 
to affect its bottom. The motion in these parts during the winter is, therefore, very 
small. 

In this view, also, may be found an explanation of the movement of glaciers 
for considerable distances up-slope, even when the surface of the ice, as well as its | 
bed, is inclined backwards. So far does this go, that a few superglacial streams 
run for some distance backwards, i. e., toward the heads of the glaciers, while in 
other places surface waters are collected into ponds and lakelets. Such a slope 
of the surface of ice is not difficult to understand if the movement is due to thrust 
from behind, or if it is occasioned by internal crystalline changes acting on a rigid 
body; but it must be regarded as very remarkable if the movement of the ice is 
that of a fluid body, no matter how viscous, for the length of the acclivity is in some 
cases several times the thickness of the ice. Crevassing and other evidences of 


146 WORK OF SNOW AND ICE 


brittleness and rigidity find a ready elucidation under the view that ice is really 


a solid body at all times, and that its apparent fluency is due to the momentary ~ 


fluidity of small portions of its mass assumed in succession as compression demands. 

In addition to the considerations already adduced, it may be urged that a 
glacier does not flow as a stiff liquid because its granules are not habitually drawn 
out into elongated forms, as are cavities in lavas, and plastic lumps in viscous 
bodies. Flowage lines comparable to those in lavas are unknown in glaciers. 

All this is strictly consistent with our primary thesis, that a glacier is crystalline 
rock of the purest and simplest type, and that it never has other than the crystalline 
state. This strictly crystalline character is incompatible with viscous liquidity. 

Other views of glacier motion. While these views of glacial motion seem 
to us to accord best with the known facts, they are not to be regarded as established 
in scientific opinion, or as the views most commonly held. The main alternative 
interpretations that have been entertained are the following: 

(1) In the early days of glacial studies De Saussure thought that glaciers 
slid bodily on their beds. 

(2) Charpentier and Agassiz referred the movement to the expansion of 
descending water freezing within the glacier. 

(3) Rendu and Forbes, followed by many modern writers, believed ice to be 
viscous, and that in sufficiently large masses it flows under the influence of its own 
weight, like pitch or asphalt. 

(4) Others, realizing the fundamental difference between crystalline ice and 
a true viscous body, have fallen back on a vague notion of plasticity, which scarcely 
amounts to a definite hypothesis at all. 

(5) Tyndall urged that the movement was accomplished by minute repeated 
fracturing and regelation, appealing to the fact that broken pieces of ice slightly 
pressed together at melting temperatures freeze together, but neglecting the fact 
that this would destroy the integrity of the crystals. 

(6) Moseley assigned the movement to a bodily expansion and contraction 
of the glacier, analogous to the creeping of a mass of lead on a roof. 

(7) James Thompson demonstrated that pressure lowers the melting-point. 
and while this effect is so small as probably to be ineffectual, it is correlated with 
the very important fact that compression, by generating heat, may cause melting, 
which is not the case in most other rocks. He recognized that under pressure 
partial liquefaction took place, that the water so liberated might be refrozen as 
it escaped from pressure, and appears to have regarded this as a vital factor. 

(8) Croll held that the movement was due to a consecutive series of molecular 
changes somewhat like the chain of chemical combinations in electrolysis. 

(9) Hugi, Eli de Beaumont, Bertin, Forel, and others thought that the 
growth of the granules was the leading factor in ice movement. 

(10) McConnel and Miigge have made the gliding planes of the ice crystals 
serve an important function in glacial movement. 

It will be seen that the principle of partial liquefaction for which Thompson 
laid the basis, the crystallization of descending water urged by Charpentier and 
Agassiz, and the granular growth on which Hugi, Beaumont, Forel, and others 
founded their hypotheses, are incorporated in the view already presented. Prob- 
ably the agencies on which some of the other views are based may also be partici- 
pants in producing glacial motion, in some places as incidental factors, and in others 
perhaps as important ones. 


EROSION BY GLACIERS 144 


THE WORK OF GLACIERS 


Erosion 


Glaciers abrade the valleys through which they pass, carry for- 
ward the material which they remove from the surface, and wear, 
grind, and ultimately deposit it. Like other agents of gradation, 
their work includes erosion, transportation, and deposition. 

Getting load. If the snow-field which is to become a glacier 
accumulates on a rough surface covered with rock debris, the glacier 
has a basal load when it begins to move, for the snow covers, sur- 
rounds, and includes such loose blocks of rock as project above the 
general surface, and envelops all projecting points of rock within its 
field. When the ice begins to move, it carries forward this debris 
in its bottom, and tears off the weak points of rock which project up 
into it. In addition to the basal and sub-glacial load which the 
glacier has at the outset, there may be surface debris which has 
fallen on the snow or ice from cliffs above. If debris descending to 
the glacier in this way is unburied, it is superglacial, but if it has been 
buried by subsequent falls of snow, it is englacial. 

Once in movement, the ice not only moves the debris to which it 
was originally attached, but it gathers new load, partly by the rasp- 
ing effect of its rock-shod bottom, and partly by its power of pluck- 
ing off or quarrying out considerable blocks of rock from its sides 
and bottom. ‘This plucking process is at its best where the ice passes 
over cliffs of jointed rock, but is not confined to such situations. 
The steep bed of a valley glacier may be worn more by plucking 
than by rasping. The advancing ice gets some material, too, espe- 
cially loose debris, by freezing to it, for the water in the soil freezes 
and becomes’ continuous with the ice above, and moves with it. 
Superglacial material may be acquired during movement, as well as 
before it, by the fall of debris from cliffs, or by the descent of ava- 
lanches. : 

Conditions influencing rate of erosion. (1) Ice wears a flat 
surface relatively little, since there is little for it to get hold of. 
Glaciers have been known to override such a surface, burying its 
soil and more or less of its herbaceous vegetation. Erosion is at its 
maximum, so far as influenced by topography, when the surface is 
rough enough to offer notable catchment for the base of the ice, but 
not so rough as to impede its motion seriously. Other conditions 
which influence glacial erosion are (2) the amount of loose or slightly 


148 WORK OF SNOW AND ICE 


attached debris on the surface; (3) the slope of the surface; (4) the 
thickness of the ice; (5) its rate of movement; (6) the resistance of 
the rock; and (7) the amount and kind of debris the ice carries. 
The effect of most of these conditions is evident, but the last two 
call for a word of explanation. 

So far as concerns the resistance of the rock, it should be noted 
that resistance is not a matter of hardness simply. Rock which is 
affected by cleavage, whether joints or bedding planes or both, is 
eroded readily, expecially on steep slopes, even if very hard. In 





Fig. 145. Striz on bed-rock. Kingston, Des Moines County, Iowa. 


such situations, the removal of rock in large blocks (plucking) is 
probably more important, on the whole, than wear by the debris 
carried. : 

Clean ice passing over a smooth surface of solid rock would 
have little effect upon it; but a rock-shod glacier abrades the same 
surface notably. The effect of this abrasion is shown in the grooves 
and scratches (stri@) which the stones in the bottom of the ice inflict 
on the surface of the rock over which they pass (Figs. 145 and 146). 
At the same time, the stones in the ice are worn by abrasion both 
with the bottom, and with one another (Fig. 147). It does not fol- 
low, however, that erosion is greatest when there is most material 
in the bottom of the ice; for with increase of debris there may be 


EROSION BY GLACIERS 149 





Fig. 146. Strie, grooves, etc., in a canyon tributary to Big Cottonwood Can- 
yon, Wasatch Mountains. (Church.) 





Fig. 147. Stones striated by glacial wear. Theirshapes, as well as their mark- 
ings, are characteristic. 
decrease of motion,! and decrease of motion retards erosion. When 
any considerable thickness of ice at the bottom of a glacier is full 
of debris, the loaded part may approach stagnancy, while the 
cleaner ice above shears over it. A moderate but not excessive load 
of debris, therefore, favors great erosion. Something depends, too, 
on the character of the load. Coarse, hard, and angular debris is 

1 Russell. Jour. of Geol., Vol. III, p. 823. 


150 WORK OF SNOW AND ICE 


more effective for abrasion than fine, soft, or rounded material. 
In plucking, rate of motion is probably more important than load. 

So far as concerns the ice itself, erosion is not most effective at 
the end of a valley glacier, or at the edge of an ice sheet, for here 
the strength of movement is too slight and the load too great; nor 
is it most effective at the source or near it, for the ice here moves 
slowly and its load is likely to be slight. Ice alone considered, 
erosion is most effective somewhere between the source and the 
terminus of a glacier, and probably much nearer the latter than the 
former. 

In summary it may be said that rapidly moving ice of sufficient 
thickness to be working under goodly pressure, shod with a sufficient 
but not excessive quantity of hard-rock material, passing over non- 
resistant formations possessing a topography of sufficient relief to 
offer some resistance, and yet too little to retard the progress of the 
ice seriously, will erode most effectively. 

Varied nature of glacial debris. From its mode of erosion it will 
be seen that a glacier may carry various sorts of material. At 
its bottom there may be (1) bowlders which the ice has picked up 
from the surface, or which it has broken off from projecting points 
of rock over which it has passed; (2) smaller pieces of rock of the 
size of cobbles, pebbles, etc., either picked up by the ice from its 
bed or broken off from larger masses; (3) the fine products (rock- 
flour) produced by the grinding of the debris in the ice on the rock- 





Fig. 148. A mountain valley in the Wasatch Mountains, not glaciated. 
(Photo. by Church.) 


EROSION BY GLACIERS Ist 


bed over which it passes, and similar products resulting from the 
rubbing of stones in the ice against one another; and (4) sand, clay, 
soil, vegetation, etc., derived from the surface overridden. Thus 
the materials which the ice carries (called drift) are of all grades 
of coarseness and fineness, from huge bowlders to fine clay. The 
coarser materials may be angular or round at the outset, and their 
forms may be changed and their surfaces striated as they are moved 
forward. Whether one sort of material or another predominates 
depends primarily on the nature of the surface overridden. 

The topographic effects of glacial erosion. In passing through 
its valley, an alpine glacier deepens it, widens its lower part, and 
smoothes its slopes up to the limit of the ice. It tends to make a 





Fig. 149. A mountain valley which has been strongly glaciated, Wasatch 
Mountains. (Photo. by Church.) 


V-shaped valley (Fig. 148) U-shaped (Fig. 149), and to make its 
head big, blunt, and steep-sided. Such a valley head is a cirque 
(Pl. XIII). The change in topography at the upper limit of 
glaciation is striking in many places (Fig. 150). 

The deepening of a valley by glacial erosion may throw its 
tributaries out of topographic adjustment. Thus if a main valley 
is lowered 100 feet by glacial erosion while its tributary is not 
deepened, the lower end of the latter will be too feet above the 
former when the ice disappears. Such valleys, called hanging val- 
leys (Fig. 151), are common in the western mountains of North 
America which were recently glaciated. 


152 WORK OF SNOW AND ICE 


Ice-caps which overspread the surface irrespective of valleys 
and hills tend to reduce angularities of surface. Hills and ridges 
are cut down and smoothed (Figs. 152 and 153); but since valleys 





Fig. 150. Contrast between a glaciated rock surface below, and non-glaciated 
crests above. Kearsarge Pinnacles, Bubbs Creek Canyon, Cal. 


parallel to the direction of movement are deepened at the same 
time, it is doubtful if the relief is commonly reduced by the erosion 
of an ice-cap. 

Fiords. A valley glacier descending to the sea may gouge out 





Fig. 151. A hanging valley near Lake Kootenay. (Photo. by Atwood.) 


TRANSPORTATION OF DEBRIS 153 


the head of a bay or the lower end of a valley to a very considerable 
depth. When the ice melts, the bay, if narrow, deep, and long, 
with high slopes, is called a fiord. Many of the fiords of coasts in 
high latitudes originated in this way, and some glaciers of. these 
coasts are now mak- 
ing fiords. Sinking 
accompanying or 








44 
erases Pere ey << 


following glaciation, 
is also a factor in . Fig. 152. Diagram representing a hill unworn by 
the makin go Photds ice, and the irregular contact of soil and rock. 

The positions 


in which debris is 
carried. Debris is 
carried in three po- 
sitions: (1) basal or 
subglacial, (2) engla- 
cial, and (3) super- 
glacial. ‘The material picked up or rubbed off from the surface 
over which the ice moves is normally carried forward in the bottom 
of the ice, and is therefore basal; that which falls on the surface is 
usually carried there, and is therefore superglacial. Either basal or 
super-glacial drift may become englacial. The basal load of a glacier 
is constantly being mixed with new drift from the ground over which 
the ice is passing. The superglacial material, on the other hand, 
may be borne from its place of origin to its place of deposition with- 
out such intermixture. 

Transfers of load. Superglacial debris obviously may become 
englacial or basal by falling into crevasses, or by being carried down 
by descending waters. 

Debris which is basal at the outset, may become englacial or super- 
glacial later. Thus when ice passes over a hill, the bottom of the 
ice rends debris from 
Merlo 10 the ‘lee 
of the hill the ice 
from either side may 
close in under that 
which came over the _ Fig. 154. Diagram illustrating one way in which a 
top. The debris de- glacier gets englacial material. 
rived from the top of a hill by the bottom of the overriding ice 
will then be well up in the ice (Fig. 154). 








Pes EF ars EE we — 
yg Se es =a 
en ee SS SS SS ae Pe 





‘Fig. 153. Diagram showing the effect of glacial 
wear on a hill such as is shown in Fig. 152. 





154 WORK OF SNOW AND ICE 


Englacial drift may become superglacial by surface ablation. 
In this case the drift does not rise, but melting brings the surface of 
the ice down to it. This occurs chiefly at the end or edge of the 
ice, where the surface melting is greatest. Englacial debris, es- 
pecially that near the bottom, also may become basal by the 
melting of the ice beneath it. 

Drift is sometimes transferred from a basal to an englacial and 
then to a superglacial position by upward movement. Such trans- 





Fig. 155. Taking debris from a protuberance of the bed. 


fer is the more remarkable because the specific gravity. of rock is 
about three times that of ice, so that the normal tendency of rock 
is to sink in ice. In arctic glaciers, and probably in others, some 
material which has been basal becomes englacial by being sheared 
forward over ice in front of it. So far as observed, this takes place 
chiefly where the ice in front of the plane of shearing lies at a lower 
level than that behind, as where the surface of an upland falls off 
into a valley, or where a boss of rock shelters the ice in its lee from 
the thrust of the overriding ice (Fig. 155). 

At the borders of many arctic glaciers the lower layers are turned 
up, as shown in Fig. 156. Where the layers turn up at the end of a 


TRANSPORTATION OF DEBRIS 155 





, 





Fig. 156. End of a North Greenland glacier, showing the upturning of the 
layers of ice at the end. This structure is common in North Greenland. At one 
point, a few stones are seen on the surface of the ice where an upturned layer comes 
to the surface. 


glacier, basal and englacial debris are carried to the surface by actual 
upward movement, and a terminal moraine or a series of terminal 
moraines may be developed where the upturned layers of ice outcrop 
at the surface (Fig. 157). That the material of these moraines was 
originally basal is shown in many cases by the bruised and scratched 





__ Fig. 157. Surface terminal moraines due to upturning. Edge of the ice-sheet, 
North Greenland. 


condition of the bowlders and pebbles, or by the nature of the mate- 
rial itself. The upturning may affect the edges of glaciers (Fig. 158) 
as well as their ends, and the material thus brought to the surface 
gives rise to lateral moraines. In some cases, too, there is upturn- 
ing of the ice along a longitudinal zone well back from the lateral 
margins (Fig. 158), and the material brought to the surface in such 
a zone giyes rise to a medial moraine. This upturning of ice has 


156 WORK OF SNOW AND ICE 


been observed only at or near the terminus of the ice. It perhaps is 
due in part to the resistance of frozen morainic or other material 
beneath and in front of the edge. ‘To this should probably be added 
the effect of the great rigidity of the outer part of the ice due to the 
low external temperature during the larger part of the year, while 





Fig. 158. Diagram to illustrate one method of formation of medial and lateral 
moraines. The horizontal line at the base represents sea-level, and the lower 
part of the glacier is under the sea. The layers of upturning ice bring debris up 
along the planes of movement, and it accumulates at the top as indicated. 


the interior, with its higher temperature, remains more fluent; 
but even this probably leaves the explanation incomplete. 

Wear of drift in transit. Drift carried at the bottom of the ice 
is much worn, for the materials in transit abrade one another and 
are abraded by the bed over which they pass. Englacial drift is 
subject to less wear, because it commonly is more scattered. Super- 
glacial drift is worn little or none while it lies on the surface of the 
ice; but in so far as superglacial or englacial drift is derived from 
basal drift, it may show the same evidences of wear as the basal 
drift itself. In many cases superglacial drift reveals its history in 
this way. 

Deposition 

During the advance of a glacier, deposition takes place both (1) 
beneath the body of the ice, and (2) beneath its end and edges. In 
the former position it takes place where the topography favors 
lodgment, or where the ice is overloaded. The topography favoring 
deposition is much the same as that favoring erosion, but the two 
processes are not favored at the same points. Erosion is greatest 
on the ‘‘stoss”’ side (the side against which the ice advances) of an 
obstruction, and deposition on the lee side (Fig. 159). Glacier ice 





Fig. 159. Crag and tail. The passage of glacier ice is likely to leave drift in 
the lee of the boss of rock, C. 


DEPOSITION OF DRIFT 157 


is likely to be overloaded (1) just beyond a place where conditions 
have favored erosion, and (2) where the ice is thinning rapidly.- On 
the whole, deposition beneath the body of a glacier back from its 











iS 


Fig. 160. Glacier building an embankment. Southeast side of McCormick 
Bay, North Greenland. 
end or edge, is much more than balanced by erosion in the same 
position. 

At and near the end of a glacier, deposition goes on faster than 
elsewhere, chiefly because of the rapid melting, and therefore the 
thinning and weakening of the ice. If the end of the glacier is 
stationary in position, drift is being brought to it continually and 








Se NS ok LS aah 





Fig. 161. Embankment completed. Near the last. (Fig. 160.) 


158 WORK OF SNOW AND ICE 


left there, for it is to be remembered that the ice is moving, though 
its end is stationary. Ifa glacier moves forward 500 feet per year, 
while its end is melted at the same rate, all the debris in the 500 
feet of ice melted, is deposited, and all except that washed away 
is deposited at and beneath the end of the glacier (Figs. 160-161). 
Uf ice advances 500 feet per year and is melted back 600 feet in the 





Fig. 162. Illecillewat Glacier; Glacier, British Columbia. A lateral moraine 
at the right of the ice records its diminution. 


same time, all the debris carried by the 600 feet melted has been 
deposited, and largely in the narrow zone (100 feet) from which the 
ice has receded. If the end of a glacier advances 500 feet per year 
while it is being melted but 400 feet, all the drift in the 4oo feet 
melted is deposited, chiefly at or beneath the immediate margin of 
the ice. To the marginal and sub-marginal accumulations made 
in this way, the material carried on the ice is added whenever 
the ice is melted from beneath it. Deposition beneath the lateral 


GLACIAL DEPOSITS 159 





F ig. 163. The moraines about the lower end of a glaciated mountain valley. 
Bloody Canyon, Cal. (U.S. Geol. Surv.) 


margins of a glacier is much the same as beneath its terminus 
(Fig. 163). 


Types of Moraines 


The terminal moraine. The thick accumulation of drift made 
at the end of a glacier or at the edge of an ice sheet, especially where 
its end or edge is stationary or nearly so for a long time, is the 
terminal moraine. ‘Terminal moraines of ice caps are of more im- 
portance, relatively, than those of valley glaciers, for streams are 
more effective in destroying the moraines of the latter. The topog- 
raphy of terminal moraines is rather distinctive, as illustrated by 
Fig. 168. | 

The ground moraine. When a glacier disappears, all its debris 
is deposited. All drift deposited beneath the body of the ice, and 
all deposited from its base during dissolution, constitutes the ground 
moraine. ‘The thickness of the ground moraine is notably unequal. 
In general, it is thicker toward the terminus of the glacier and 
thinner toward its source, but considerable portions of a glacier’s 
bed may be left without debris when the ice melts. As a rule, the 
ground moraine is thinner than the terminal moraine, and less irreg- 
ularly disposed. The ground moraines of valley glaciers are rela- 
tively unimportant as compared with those of ice-caps, since condi- 
tions for erosion under the body of a valley glacier are, on the 
average, better than under an ice-sheet, while those for deposition 


160 . WORK OF SNOW AND ICE 


are less favorable. The topography of the ground moraine (Plate 
XIV) is, as a rule, less uneven than that of the terminal moraine 
(Fig. 168). 

Lateral moraines. Lateral moraines are the product of valley 
glaciers. The lateral moraines on such glaciers are let down on 
the surface beneath when the ice melts; but the lateral moraines 
in a valley from which the ice has melted are not merely the lateral 
moraines which were on the glacier. They are made up chiefly of 
drift accumulated beneath the sides of the glacier. This accumula- 
tion is the result of the lateral motion of the ice from the center to 





Fig. 164. A lateral moraine lett by a former glacier in the Bighorn Mountains 
of Wyoming. (Photo. by Blackwelder.) 


the sides of the valley. Such sub-lateral accumulations are akin to 
terminal moraines. Some of the lateral moraines of ancient valley 
glaciers, those like of the Uinta, Wasatch, and Bighorn mountains 
are several hundred feet high (Fig. 164), or even as much as a thou- 
sand. In northern Italy a lateral moraine is said to be more than 
2,000 feet high. 

Distinctive nature of glacial deposits. The deposits made by 
glaciers are distinctive. In the first place, the ice does not assort 
its drift, and bowlders, cobbles, pebbles, sand, and clay are con- 
fusedly commingled (Fig. 165). In this respect, the deposits of ice 
differ notably from those of water. Furthermore, many stones of 
the drift show the peculiar type of wear which glaciers inflict. 

* Geikie. The Great Ice Age, 3d ed., p. 529. 


GLACIAL DEPOSITS 161 





Fig. 165. Section of drift showing its heterogeneity: 

Though notably worn, they are not rounded like the stones carried 
by rivers. Many of them have sub-angular forms with planed and 
beveled faces, the planes being striated and bruised (Fig. 147). 
Absence of stratification, physical heterogeneity, and the striation 
of at least a part of the stones are among the most distinctive char- 
acteristics of glacial drift. A not less real though less obvious 
characteristic is the constitution of the fine material, for it is, as a 
rule, the product of rock grinding, not of rock decay. 

Glaciated rock surfaces. Another distinctive mark which a 
glacier leaves behind it is the- character of the surface of the rock 
-on which the drift rests. This is generally smoothed by the severe 
abrasion to which it has been subjected, and the smoothed surfaces 
(Figs. 145 and 166) are marked by grooves and striz, similar to 
those on the stones of the drift (Fig. 147). Other distinctive fea- 
tures of a glaciated area are rounded bosses of rock (roches mouton- 


162 WORK OF SNOW AND ICE 





Fig. 167. Roches moutonnées, Engineer Mountain, Colo. (Hole, U.S. Geol. 
Surv.) 


GLACIAL TOPOGRAPHY 163 


S) 


nées, Fig. 167), rock basins, ponds, and marshes, and the peculiar 
topographies resulting from the unequal erosion (Pl. XIII), and 
the still more unequal deposition (Fig. 168) of drift. Surface bowl- 











Fig. 168. Sketch of drift (terminal moraine) topography near Hackettstown, 
N. J. (N. J. Geol. Surv.) 
ders, in many cases unlike the underlying formations of rock, and 
sometimes in peculiar and apparently unstable positions (perched 
bowlders) are still another mark of a glaciated area (Fig. 169). 





Fig. 169. Perched bowlder, New Jersey. 


164 WORK OF SNOW AND ICE 


GLACIO-FLUVIAL WORK 


The streams to which the melting of the ice gives rise are laden 
with gravel, sand, and silt derived from the ice. Where the mud 
is light-colored, the streams are sometimes described as ‘‘milky.”’ 
Where the amount of material carried is great, much of it is dropped 
at a slight distance from the ice, the coarsest being dropped first. 
Glacial streams are, as a rule, aggrading streams, and therefore 
develop alluvial plains, called valley trains (Fig. 170), or, where they 





Fig. 170. Diagram to illustrate the profile of a valley train, and its relations 
to the terminal moraine (m) in which it heads. 


enter lakes, bays, or other streams, deltas. In its transportation, 
the river-borne drift is assorted, and after its deposition it is strati- 


ite aS Sere Wea 





>.” 


Fig. 171. Esker of Punkaharju, Finland. 





fied. Glacial deposits in the upper part of a mountain valley are, 
therefore, generally connected with glacio-fluvial deposits farther 
down the valley. The silt, sand, and gravel of valley trains can, 
as a rule, be distinguished from valley deposits of non-glacial origin 









PLATE XIII 


GH) AN S APS ERK Wi 
Aa AW WY FS))/S 


won 





9 

== 
oD 

ee | . 


= 


sy 







Z, ee 





{YA 


oA, 









a 






| shi} Jp J, . Uy Hi 
7 ‘Sj fe GSK SOG a A WY —FBESS BLN YY 
f ; 
= 










A portion of the Bighorn Mountains, showing glaciated valleys, the heads of 
which are in many cases cirques. Scale, about 2 miles per inch. 
(Cloud Peak, Wyo., Sheet, U. S. Geol. Surv.) 


PLATE XIV 





Characteristic surface of a glaciated plain, showing marshes, ponds, 
and lakes. Southern Wisconsin. Scale, about 1 mile per inch, 
(Silver Lake, Wis., Sheet, U. S. Geol. Surv.) 


oe a 


GLACIO-FLUVIAL DEPOSITS 165 


by the fact that they are largely of undecayed rock material, espe- 
cially if deposited recently. 

Numerous streams flow from an ice-sheet, spreading their debris 
in front of the terminal moraine, forming a broad fringing sheet ‘of 
gravel and sand (outwash plain) along it. Outwash plains have 
much in common with piedmont alluvial plains. They differ from 
valley trains chiefly in being shorter, wider, and not confined to 
valleys. 

Where streams of considerable size form tunnels under the i ice, 
the tunnels may become more or less filled with water-worn debris, 
and when the ice melts, the aggraded channels appear as ridges of 
gravel and sand, known as eskers (Fig. 171). It has been thought 
that eskers represent deposits formed in superglacial channels; but 
this is probably rarely if ever the case, for most surface streams have 
high gradients, swift currents, and smooth bottoms, and hence give 
little opportunity for lodgment. Furthermore, ice-sheets, in con- 
nection with which eskers are developed, have no surface material 
except at their immediate edges. 

At the mouths of ice-tunnels or ice-channels, and in the re-entrant 
angles of the edge of the ice, sands and gravels are liable to be 
bunched in quantity, giving rise, after the adjacent ice has melted, 
to peculiar hills and hollows of knob-and-basin type. The hills 
and short ridges of stratified drift formed in this way are known as 
kames. Much stratified drift (gravel, sand, and silt) deposited by 
glacial streams has no distinctive topographic form, and therefore 
no special name. 

All fluvio-glacial deposits are stratified. Kames and eskers 
made in immediate association with the ice, and more or less affected 





Fig. 172. The end of a glacier in Spitzbergen. (Rabot.) 


166 WORK OF SNOW AND ICE 


by its movements, are less perfectly and regularly stratified than 
valley trains and outwash plains. 


ICEBERGS 


Where glaciers advance into water the depth of which approaches 
their thickness, their ends are broken off (Fig. 172), and the de- 
tached masses float away as icebergs (Fig. 173). Many of the bergs 


ih 





‘of MANE 


Fig. 173. Aniceberg. (Robin.) 


are overturned, or at least tilted, as they set sail. If this does not 
happen at the outset, it may later, as the result of melting, wave- 
cutting, etc., which shift the centers of gravity of the bergs. The 
great majority of them do not float far before losing all trace of 
stony and earthy debris; but the finding of glaciated pebbles in 
dredgings far south of all glaciers shows that bergs occasionally 
carry stones far from land. The importance of icebergs as agents 
of transportation has been greatly exaggerated, and the assignment 
of shoals, like the Banks of Newfoundland, to them, is without 
foundation. 


Map work. See Interpretation of Topographic Maps, Exercises XI to XIII, 
and Plates XCV to CXXIX, Professional Paper 60, U. S. Geological Survey. 


CHAPTER VI 
THE WORK OF THE OCEAN 


A few facts concerning the depth of the ocean and the distribu- 
tion of its water have been given on a preceding page (p. 5), and 
reference to the origin of the ocean basins and the ocean will be 
made later. We are concerned here chiefly with the geologic proc- 
esses now going on in the sea; but a few facts concerning the sea- 
water and its life, and the topography of the ocean’s bed,! may 
well precede the study of the processes now in operation. 

Mineral matter in solution. Every 1,000 pounds of sea-water 
contain about 34.40 pounds of mineral matter in solution. The 
principal substances in the water are the following: 


MMT CP GS he ye ee un BS a theres vy OG Cte: 
MMIC STIAULCSIUIN : ¢ 25... dy cw ey tte eee eee eens 10.878 
PMMIPEMINTI AGIOS ye. or ae fa eM eben es el 4.737 
LEER CPR ep, OU de eS a 3.600 
IMIS OL MSSINTSN Slee 5 clef be bere a. gots oe oe wala aes ope ys 2.465 
RE LECTION te Ca ay ih cic sie tldices ch wa vsive 0.345 
SeEEPIIEE MA OTCSPUNT ey os. a Se Ad eve honk eh ole hae ete Ocary 


There are many other mineral substances in sea-water, and 
the gases oxygen, nitrogen, and carbon dioxide are present in quan- 
tity. -The amount of the last is estimated to be 18 times that in 
the atmosphere. 

The amount of sea-water is estimated at about 324,000,000 
cubic miles, or about 15 times the volume of the land above sea- 
level. The volume and composition of the sea-water being known, 
the amount of its mineral matter may be calculated. Assuming 
the average specific gravity of the mineral matter to be 2.5, the 
3.5% (nearly) by weight becomes about 1.4% by volume, and 1.4% 
of 324,000,000 cubic miles is 4,536,000 cubic miles. This represents 
approximately the volume which the mineral matter of the sea 


1 Much information on these and other points is to be found in the following 
books: Wild’s Thalassa; Thompson’s Depths of the Sea; Barker’s Deep Sea 
Soundings, and Agassiz’s The Three Cruises of the Blake. ‘The Challenger Reports 
give more detailed information for certain regions. 

2 Dittmar, Challenger Reports, Physics and Chemistry, Vol. I, p. 204. 


167 


168 WORK OF THE OCEAN 


would have if it were precipitated and compacted so as to have an 
average specific gravity of 2.5. This amount of mineral matter 
would cover the ocean bottom to a depth of about 175 feet. Its 
amount is equal to about 20% of that of all lands above sea-level. 

A large part of the mineral matter of the sea has come from the 
land, where it was dissolved chiefly by ground-water, and carried 
to the sea by rivers. But the mineral matter of the sea gives no 
more than a hint of the importance of the solvent work of water 
in the general processes of rock decay, for most of the mineral 
matter carried from the land to the sea in solution is taken from 
_sea-water about as rapidly as it is received. Calcium carbonate, 
for example, is about twenty times as abundant as sodium chloride 
in river-water, but it is only 1/200 as abundant in sea-water. This 
is because the calcium carbonate is used by animals and plants to 
make shells, skeletons, etc., while the salt remains in solution. 

From the amount of water discharged by rivers into the sea each. 
year (about 6,500 cubic miles), and from the amount of salt it carries, 
it is calculated that it would take about 370,000,000 years for the 
salt of the sea to have been contributed by rivers, at the present rate. 
This figure, however, must not be taken as the age of the ocean, for 
(1) the salt is not all brought in by rivers, (2) it is not probable 
that the rivers have always contributed salt at the present rate, 
and (3) much salt once in the sea has been precipitated. Never- 
theless the above figure gives some suggestion as to the order of 
magnitude of the figure which represents the age of the ocean. 

Topography of ocean basins. The ocean basins are convex 
upward. It is only when we remember that a level surface (6n the 
earth) is one which has the mean curvature of the earth, and that 
the deeper parts of the ocean basin are considerably below the 
mean sphere level, that the name basin seems appropriate. 

The bed of the ocean, like the face of the land, has elevations 
and depressions, and its deepest parts are about as far below its 
surface as the highest mountains are above it. If the water were 
drawn off, so that the bottoms of the ocean basins could be seen, 
three great features would appear: (1) Extensive tracts of low land 
(now covered by deep water); (2) other great, but less extensive 
tracts of higher land (now covered by shallow water); and (3) 
ridges and peaks of mountainous heights. These three principal 
divisions may be compared to the plains, plateaus, and mountains 
of the land, though mountain systems would be less numerous than 


GENERAL FEATURES 169 


on land. In addition there are great depressions comparable to the 
great basins of the land. 

Apart from these general features, there is little in common be- 
tween the topography of the sea bottom and that of the land. If 
the ocean’s bed could be seen as the land is, its most impressive 
feature would be its monotony. The familiar hills and valleys 
which give the land its most familiar features are essentially absent. 
A large part of the ocean bottom is so nearly flat that the eye would 
not detect its departure from planeness. 

The reason for this difference is readily found. The dominant 
processes which shape the details of the surface of the land are 
degradational, and though the final result of degradation is flatness 
(base-level), the earlier result is roughness. In the sea, the domi- 
nant processes are aggradational, and tend to planeness. 

Distribution of marine life. Marine life has been of such im- 
portance in the history of the earth that the elementary facts 
concerning its distribution and the principles which control it are 
here recalled. Its distribution is influenced by many factors, chief 
among which are temperature and depth of water. It is more abun- 
dant in the warmer parts of the ocean than in the colder, the species 
inhabiting cold waters are different from those in warm, and few 
species range through great variations of temperature. Many 
forms are restricted to shallow water; many others, especially those 
living near the surface, swim about freely without reference to 
depth; while a few are restricted to great depths. Some species 
are influenced by (1) the salinity of the water, which varies con- 
siderably along coasts where the fresh waters from the land are dis- 
charged; (2) the character of the sediment at the bottom, some species 
preferring mud, others sand, etc.; (3) the movement of the water, 
some species preferring quiet water and others rough water; (4) 
the abundance and nature of the food-supply; and (5) the presence 
or absence of rival and hostile species. 

Subject to exceptions determined by temperature, etc., plant 
life abounds in the superficial parts of the ocean, and down to the 
bottom where the depth does not exceed too fathoms. Animal 
life is abundant in shallow water at all depths down to 200 or 300 
fathoms, and in the surface-waters of temperate and tropical regions 
regardless of depth. The great body of the ocean-water lying below 
the depth of a few hundred fathoms has but little life, though 
animals exists sparingly at the bottom, even where the depth is great. 


170 WORK OF THE OCEAN 


PROCESSES IN OPERATION IN THE SEA 


Diastrophism. So far as the lithosphere is concerned, the sea~ 
level is the critical level. At this level and above, many processes 
are in operation which are not effective below, while below sea-level 
some processes are effective which find no counterpart above. 
Warpings of the surface which do not involve the submergence of 
land or the emergence of sea bottom, are relatively unimportant 
compared with those which do. The rise of the bottom of the sea 
from a depth of 400 fathoms to a depth of 200 fathoms would not 
have important results, so far as the area itself is concerned, while 
an equal rise of the bottom beneath too fathoms of water, or an 
equal sinking of land 500 feet high, would be much more important. 





Fig. 174. Map showing the early stages in the simplification of a shoreline 
by deposition, and showing that at this stage the irregularities are increased. | 
If the land rose or the sea sank 100 fathoms, the coast-line would be regular. 


PROCESSES IN OPERATION iy ge 


{t follows that the changes effected by diastrophism are more obvious 
in shallow water than in deep. Emergence or submergence shifts 
the zone of contact of ocean and land, and so the areas of aggrada- 
tion and degradation, and changes the region concerned from one 
appropriate for sea life to one appropriate for land life, or vice versa. 

Over the continental shelves the water is shallow and the bottom 
relatively smooth. If the sea-level were drawn down, or if the con- 
tinental shelf were elevated evenly, the new shore-line on the smooth 
surface of the former submerged shelf would be regular relatively, 
even though the coast was notably irregular before the change. 
This is illustrated by Fig. 174. Subsidence of a coast-line (or rise 
of the sea-level) tends to the opposite result, for in this case the 
sea advances on a surface which has relief, and the water covers 
every low place sunk to its level. Thus the numerous bays at 
the lower ends of the streams along the Atlantic coast from Long 
Island Sound to Carolina are the results of recent sinking. From 
the present configuration of coast-lines it has been inferred that the 
present is an era of continental depression. Some river valleys, the 
lower ends of which are embayed, are found to be continuous with 





15) Lol 

Fig. 175.. The ecbierced valley which has been interpreted as the continua- 
tion of the Hudson Valley. The position of the valley is indicated by contours. 
(Data from C. and G. Survey.) 


172 WORK OF THE OCEAN 


submerged valleys beyond the coast-line (Fig. 175). Submerged 
river valleys show that the surface in which they lie was once land. 

The effects of diastrophism in the ocean and about its borders, 
may (1) make the water of any ocean, or of any part of it, shallower 
or deeper; (2) cause the emergence or submergence of land; (3) make 
coast-lines regular or irregular; (4) shift the habitat of many forms 
of life, and, through these changes, (5) influence the processes of 
gradation, especially at and near the contact of sea and land. 

Vulcanism. Vulcanism affects the sea-bottom much as it affects 
land. At the volcanic centers, where the great body of extruded 
matter accumulates, mounds and mountains are built up, and most 
of the mountain peaks of the sea-bottom had a volcanic origin. 
Where volcanic cones are built up near the surface of the sea, they 
may furnish a home for shallow-water life, such as polyps. Wher- 
ever they are built up so as to be within the reach of waves, grada- 
tional processes are stimulated. 

The number of active volcanoes on islands is about 200, but the 
number of active vents beneath the sea is unknown. A few sub- 
marine eruptions have been observed, but those observed are prob- 
ably but a small percentage of those which take place, for eruptions 
in deep water may not be seen at the surface. 

Oceanic volcanoes affect both the temperature and the composi- 
tion of the sea-water. Both the increase of temperature and the 
volcanic gases increase the solvent power of water, and both the 
change in temperature and composition affect the life of the adja- 
cent waters. Volcanoes in the sea have furnished much of the 
sediment now found on the bottom of the ocean. Some of it is very 
fine, like volcanic dust, and some of it is coarse. Both the fine and 
the coarse are distributed far from the volcanoes which emit them, 
are found indeed nearly everywhere on the bottom of the deep sea, 
though not in uniform abundance. It is therefore clear that the 
effects of oceanic volcanoes on the sea-water are considerable, when 
long periods of time are considered. 

Gradation. The gradational processes of the land and the sea 
are in striking contrast. On the land, degradation predominates, 
and aggradation is subordinate;.in the sea, aggradation predom- 
inates and degradation is subordinate. On the land, degradation 
is greatest, on the whole, where the land is highest, while aggrada- 
tion is of consequence only where the land is low, or where steep 
slopes give place to gentle ones. In the sea, degradation is vir- 


MOVEMENTS OF SEA-WATER 173 


tually confined to shallow water, or to what might be called the 
highlands of the sea, while aggradation is nearly universal, though 
most considerable in shallow water, or where shallow water gives 
place to deep. Both the degradational and aggradational work 
of the sea are greatest near its shores. Though the gradational 
work on the land and in the sea are in strong contrast, they tend 
to a common end — the leveling of the surface of the lithosphere. 

The gradational processes of the sea-bottom are effected (1) 
by mechanical, (2) chemical, and (3) organic agencies. Mechanical 
gradation is effected chiefly by the movements of the water. These 
may be degradational where the water is shallow enough for the 
motion to affect the bottom, but elsewhere they are aggradational. 
Gradation by chemical processes is likewise partly degradational 
and partly aggradational. In lagoons and other small inclosures, 
the water may become saturated with mineral matter; with further 
evaporation, precipitation takes place, the precipitate accumulating 
as sediment on the bottom. On the other hand, solution results in 
degradation. Organic agencies are, on the whole, aggradational. 
Accumulations of coral, coral debris, shells, etc., help to build up the 
sea-bottom. In the aggradation effected directly by organic 
agencies, the sea is passive. Its only part is to support the life which 
produces the solid matter, and incidentally to float a part of it in its 
currents. 

MOVEMENTS OF SEA-WATER 

The movements of sea-water fall into several categories. There 
is (1) a general circulation of sea-water, determined by (a) differences 
in density in the sea-water, (b) differences of level, and (c) move- 
ments of the atmosphere; (2) periodic tidal movements; and (3) 
aperiodic movements due to earthquakes, volcanic explosions, land- 
slides, etc. 

For present purposes, all movements of the sea-water may 
be grouped into two main classes — (1) waves, with the undertow 
and the littoral currents they generate, and (2) ocean-currents. 


W aves 
Wave-motion.! The most common waves are those generated 
by winds. During the passage of a wave, each particle affected by 


1 In the following pages concerning the waves and their work, Gilbert’s discus- 
sion of shore features, in the Fifth Annual Report of the U. S. Geol. Survey, pp. 80- 
100, is freely drawn on. See also Fenneman, Jour. of Geol., Vol. X, pp. 1-32. 


174 WORK OF THE OCEAN 


it rises and falls and moves forward and backward, describing an 
orbit in a vertical plane. If the passing wave is a swell, the orbit 
of the particle is a circle or an ellipse; but in the case of a wind-wave 
the orbit is not closed, for in such a wave the water, as well as the 
undulation, moves forward. On the crest of the wind-wave each 
particle of water moves forward, and in the trough it moves less 
rapidly backward, and the excess of the forward movement over 
the backward gives the water a slight advance. As a result of this 
advance, the upper part of the water is carried forward with refer- 
ence to that below, in the direction toward which the wind blows. 
The waves of any considerable or long-continued wind, therefore, 
generate asurface movement 
in the direction of the wind. 

Wave motion is prop- 
agated downward indefi- 
nitely, but the amount of mo- 
tion diminishes rapidly with 
increasing depth (Fig. 176). 
Engineering operations have 
; shown that submarine struc- 

Fig. 176. Diagram illustrating the de- tures are little disturbed at 
creasing size of orbits of water particles in a depths of five meters in the 
wave, with increasing depth. Mediterranean, and eight 
meters in the Atlantic. On the other hand, debris as coarse as 
gravel, which is transported by rolling on the bottom, may be 
carried out to depths of 50 feet, and sometimes even to 150 feet. 
Fine sediment, like silt, is disturbed at still greater depths, for 
ripple-marks, which indicate agitation of the water, are said to 
have been found at depths of too fathoms. 

When a wave approaches a shelving shore, its habit is changed. 
The velocity of the undulation is diminished, while the velocity of 
the advancing particle of water in the crest is increased; the wave- 
length, measured from trough to trough, is diminished, and the 
wave-height is increased; the crest becomes acute, with the front © 
steeper than the back, and these changes culminate in the breaking 
of the crest when the undulation proper ceases. Waves of a 
given height break in about the same depth of water, and the line 
along which incoming waves break is the line of breakers. The 
line of breakers is in deepest water and farthest from shore when 
the waves are strongest. The return of the water thrown forward 





MOVEMENTS OF SEA-WATER 175 





Fig. 177. Shore wave breaking on east wall of Hastings. (From Wheeler’s 
The Sea Coast; by permission of Longmans, Green and Company.) 


in the crests of waves is accomplished by a current along the bot- 
tom, called the undertow, which is sensibly normal to the coast when 
uninfluenced by oblique waves. 

When waves advance on shore obliquely, a shore-current is 
developed as illustrated by Fig. 178, where ad represents the direc- 
tion of the incoming wave, bc the direction of the shore (or littoral) 
current, and bd the direction of the (4 
undertow. Where they strike the bor- 
ders of land, the wind-waves, there- 
fore, generate two other movements, 
the undertow and the littoral current. 
Any particle of water near shore may 
be affected by any two or by all three 
of these movements at the same mo- 
ment. The effect of littoral current 
and undertow is to give a particle of 
water on which both are working a 
direction between the two, as be. The 
effect of other combinations is readily 
inferred. These various combinations : : 
are of consequence in the transporta- aA cerapieteea ima penne 
tion of debris. Waves and the move- undertow, and shore-current. 


Lda 









LMM 





Wb 


\N 


i 


176 WORK OF THE OCEAN 


ments to which they give rise (1) wear the shores, (2) transport 
the products of wear, and (3) deposit the transported materials. 

Erosion. In the dash of the waves against the shore, the wear 
is effected chiefly by the impact of the water and of the debris which 
the water carries, but lesser results are accomplished in other ways. 

When the land at the margin of the water consists of uncop- 
solidated material, or of fragmental material but slightly cemented, 
the dash of the water is sufficient to displace or erode it. If weak 
rock is associated with resistant rock within the zone of wave-work, 
the removal of the former may lead to the disruption and fall of the 
latter, especially when weak rock is washed out from beneath strong. 
The impact of the water is competent also to break up and remove 
rock which was once resistant, but which has been weakened by 
weathering. Rock affected by joints is attacked with success, for 
the blocks bounded by joints may be loosened and quarried out. 
Waves of clear water, even when their force is very great, have little 
effect on rock which is thoroughly solid. 

The effect of the impact of the waves is generally increased by 
the detritus they carry. The sand, the pebbles, and such stones 


ame 7 





ahs 


Fig.179. Angular blocks of rock, fallen from the cliff above, as a result of under- 
cutting by waves; Grand Island, Lake Champlain. 


MOVEMENTS OF SEA-WATER 177 


as the waves can move are used as weapons of attack, both against 
the shore and against one another. Masses of rock too large for 
the waves to move (Fig. 179) are worn by the detritus driven back 
and forth over them, and in time reduced to movable dimensions. 
They then become the tools of the waves, and, in use, are reduced 
still more. Thus bowlders are worn to cobbles, cobbles to pebbles, 
pebbles to sand, and sand to silt. The silt, held in suspension in 
agitated water, is carried out beyond the range of breakers, and 





Fig. 180. Showing blocks similar to those of Fig. 179, but reduced and rounded 
by wave-action. Shore of Lake Champlain. (Perry.) 


settles in water so deep as not to be agitated to its bottom. Thus 
one generation of shore bowlders after another is worn out, and the 
comminuted products come to rest in deeper water. 

The effectiveness of waves depends on their strength and on the 
concentration of their blows.!. The average force.of waves on the 
Atlantic coast of Britain has. been found to be 611 lbs. per square 
foot in summer, and 2,086 lbs. in winter, but winter breakers which 

1 Willis, Jour. of Geol., Vol. I, p. 481. 


178 WORK OF THE OCEAN 


exert a pressure of three tons per square foot are not infrequent. 
Exceptional storm-waves have moved blocks of rock exceeding 100 
tons in weight. Waves are most efficient on bold coasts bordered 
by broad expanses of deep water, for here their force is expended 
almost wholly near the water line; where shallow water borders the 
land, the force of the waves is expended over a greater area. 

The direct effect of wave-erosion is restricted to a zone which 
is narrow both horizontally and vertically. There is no impact 
of breakers at levels lower than the troughs of the waves, though 
erosion may extend down to the limit of effective agitation. The 
upper limit of effective wave-action is the level of the wave-crests. 
The rise and fall of the water during the flow and ebb of the tides 
gives the waves a greater vertical range than wind-waves alone 
would have. The indirect work of waves is limited only by the 
height of the shore, for as the zone of excavation is carried land- 
ward, masses higher up the slope are undermined and fall. The 
fallen rock protects the shore against the waves temporarily (Fig. 
179), but the fallen masses are themselves broken up eventually. 

The general result of wave-erosion is the advance of the sea on 
the land, the rate of advance being determined chiefly by the nature 
of the material attacked and the strength of the waves. Though 
examples of the retreat of coast-lines before the advance of the 










LG MYTULED 
ee Wy 
UTLEY Ms 
ANE 





Fig. 181 Fig. 182 


Fig. 181. High sea cliffs, and a submerged terrace, due partly to wave-cutting 
and partly to building. 
Fig. 182. A low sea cliff. 


sea are numerous, the advance is not universal or uninterrupted. 
On the contrary, the land encroaches on the sea in some places, and 
the two things may go on side by side. At Long Branch, N. J., 
advance of the sea has been so rapid in recent times as to menace 
important buildings, while a few miles to north and south, land is 
advancing into the sea by the deposition of shore drift. The low 
coast of the Middle Netherlands has retreated two miles or more in 
historic times, but the land has advanced at other points in the 


WAVE-EROSION 179 


same region. On the coast of England the sites of villages have 
disappeared by the advance of the sea within historic times,! but 
the coast of the same island affords illustrations of land advance. 
On the south side of Nantucket Island, the sea-cliff has been known 
to retreat before the waves six feet in a single year.?, Almost every 
considerable stretch of coast affords illustrations both of the advance 
of sea on land and of land on sea; but in the long run, the former 
exceeds the latter. 

Topographic features developed by wave-erosion. As _ the 
waves cut into the shore at and near the water-level, they develop 
a steep slope above the line of cutting. This steep slope is the sea- 
cliff (Figs. 181 to 184). The term /ake cliff is applied to the cor- 
responding cliffs of lakes. 

The height of the cliff depends on the height of the land along 
shore. Its slope may be steep or gentle (Figs. 181 and 182). Rapid 






A high sea cliff without a beach, La Jolla, Cal. 


Fig. 183. 
cutting and resistant material tend to produce steep cliffs; but 
steep cliffs may develop in incoherent materials, such as sand and 
clay, if cutting is rapid. The structure of the cliff-rock also in- 
fluences the slope and configuration of the sea-cliff. By working 
in along the joints of the rock, widening them and quarrying out 
the intervening blocks, pillars of rock. (‘‘chimney-rocks,” ‘‘ pulpit- 


1Dana, Manual of Geology, 4th ed., p. 219. 
2 Shaler, Sea and Land, p. 29. 


180 WORK OF THE OCEAN 


rocks’”’), or even considerable islets are sometimes isolated by the 
waves (Fig. 185). 
Waves may excavate caves at the bases of cliffs. The bottoms 





Fig. 185. A chimney rock and an arch on the coast of France. (Neurdein.) 


PLATE XV 


‘ (‘AINg "TOAD “Gg “AQ Yoo “ssepT 
‘Avg U0jsog) “Joo 0% ‘[VAIoJUT AN04QUOD 
‘your aed oir [ qnoqe ‘apvog  .Yyoredq,, 
vw Aq pUv[UreUt vq} 0} pot} puvyst UWY—'T ° 


0.44) Sseg 


Long Pr. 


Herring Pond 


sueyen aiyiq AM 
fa “Ais 
C 


= 
ea 
= 


*e52cet2e=% 





Fic. 2.—Coastal lakes formed by the bloc 


Contour interval, 20 feet. 


king of the ends of drowned 
(Marthas Vineyard, Mass., Sheet, U. S. Geol. Surv.) 


v 


about 1 mile per inch. 


Seale, 


valleys. 





The upper end of Seneca Lake, New York. The fiat between Montour 
Falls and Watkins is a delta which has been built out into the lake 


by the in-flowing creek. Scale, about 1 mile per inch. Contour 
interval, 20 feet. (Watkins, N. Y., Sheet, U. S. Geol. Surv.) 


WAVE-EROSION 181 


and roofs of most sea-caves have a pronounced inclination land- 
ward, and if the cliff is low, the cave may be extended landward 
until its roof is pierced. Through such an opening in the top of 
the cliff the water of the incoming waves may be forced in the form 
of spray. On the New England coast, such holes are sometimes 
known as “‘spouting horns.” Similar openings may be made by 
the compression or rarefaction of the air in the cave as the wave 
enters or retreats. Sea caves, ‘“‘spouting horns,” ‘‘pulpit-rocks,”’ 
and other isolated islets, all are closely associated with the sea-cliff 
in origin. 

The bottom of the sea-cliff is bordered by a submerged platform 
over which the water is shallow. This platform, or at any rate its 





Fig. 186. Wave-cut terrace. The land has risen or the sea sunk since the 
terrace was cut. Seward Peninsula, Alaska. (U.S. Geol. Surv.) 


landward portion, represents the area over which the water has 
advanced as the result of wave-cutting, and is known as the wave- 
cut terrace. Such a terrace is the necessary accompaniment of the 
cliff. Wave-cut terraces may become land by elevation, or by the 
lowering of the level of the sea (Fig. 186). Elevated sea-cliffs with 
wave-cut terraces at their bases are among the best evidences of 
change of relative level between water and land. 

Wave-erosion and horizontal configuration. The structure of 
the rock along shore has much to do with the horizontal configura- 
tion of the wave-shaped coast. Wave erosion develops re-entrants 
in the weaker portions of the shore, leaving the more resistant parts 
as headlands (Fig. 2, Pl. VI. p. 69). It is to be noted that the resist- 
ance of rock to wave-erosion is not determined by its hardness alone. 


182 WORK OF THE OCEAN 


Every division plane, whether due to bedding, to jointing, or to 
irregular fracture, is a source of weakness, and rock of great hard- 
ness may be so broken as to offer little resistance. A coast which is 


<j regular and of equal 
SS&& exposure, but of 
SSS unequally resistant 


Pa material, will be 


\ made irregular by 
wave-erosion. A 
regular coast of uni- 
form material, but 
unequal exposure, 
will be made irregu- 
lar by greater cut- 
ting at points of 
greater exposure. A 
coast of marked ir- 
regularity and ho- 
mogeneous material 
will be made more 
regular by the cut- 
ting off of the pro- 
jecting points, be- 
cause they are most: 
exposed. Witha 
given set of condi- 
tions, waves tend 
to develop a certain 
sort of shore-line 
which, so far as its 
horizontal form is 
concerned, is rela- 





Fig. 187. Portion of the Texas coast, showing ,. 
tendency of shore-deposition to simplify the coast line. tively stable. Such 
The deposits (narrow necks of land parallel to the a shore-line may be 


coast) shut in bays. (From chart of C. and G. Surv.) gaid to be mature: 


so far as wave-erosion is concerned. Since coastal lands are, in 
general, both heterogeneous and unequally exposed, a mature coast- 
line is somewhat irregular. 


1 Gulliver, Shore Line Topography; Proc. Am. Acad. Arts and Sci., Vel 
XXXIV, 1899, pp. 151-258. A valuable study of shore-line topography. 


SHORE DEPOSITS 183 


Since the conditions of erosion along coasts are constantly, 
even if slowly, changing, maturity is constantly being approached, 
but rarely reached. Other forces and processes, such as those of 
aggradation, vulcanism, and diastrophism, are in operation along 
coasts, and their results may antagonize the waves. The horizon- 
tal configuration of coasts is, therefore, the result of many co- 
operating forces, of which waves are but one. It is, nevertheless, 
important to note the end toward which waves are working, 
even though they are continually defeated in their attempt to reach 
it. Their immediate goal is maturity of configuration; their final 
goal is the destruction of the land and the deposition of its sub- 
stance in the sea. 

Transportation. Material eroded from the shore by waves is 
transported by the joint action of waves, undertow, and shore- 
currents. The incoming wave begins to shift material where it 
begins to drag bottom. From the line where transportation begins, 
to the line of breakers, detritus at the bottom is shifted toward the 
shore by the waves, while the undertow tends to carry it back again. 
The result of these opposed movements is to keep sediment moving 
to and from the shore in shallow water. Waves which come in at 
right angles to the shore, and the undertow resulting, do not move 
sediment along the shore; but oblique waves and littoral currents 
do. The direction in which debris is shifted by waves and shore- 
current is modified by the undertow, and the direction which would 
result from undertow and current is modified by the wave (Fig. 
178). Waves of storms, rather than those of prevailing winds, 
determine the direction of greatest transportation. 

Waves, undertow, and littoral currents work together in assort- 
ing the detritus of the shore. If the coarsest parts are beyond the 
power of all but the strongest waves, they accumulate where 
agitation is great. Less coarse parts are carried farther from the 
site of greatest agitation, but no materials which are classed as 
coarse are carried beyond the depth of sensible movement. The 
coarse materials which cover the bottom where agitation of 
the water at the bottom is effective, and which are shifted about 
by waves, etc., constitute shore drift. The material which is fine 
enough to be held in suspension is measurably independent of 
depth. This is shown during storms when the water becomes 
turbid far beyond the zone of shore drift, and clears only after 
the waves have died away. 


184 WORK OF THE OCEAN 


The sorting of shore drift, effected while it is in transportation, 
may be very perfect. The conditions favoring assortment are (r) 
vigorous wave-action, (2) prolonged transportation, and (3) a 
moderate volume of sediment. 

Deposition by waves, undertow, and shore-currents. ‘The zone 
occupied by shore drift in transit is the beach. Its lower margin 
is beneath the water, a little beyond the line where the great storm- 
waves break. Its upper margin is at the level reached by storm- 
waves, and is usually a few feet above the level of still water. Ma- 
terial is brought to the beach from seaward by incoming waves, 
and from it detritus is car- 
ye Vey, —————— ried out by the undertow. 

yy fy Wy ———_ The cross-section of a beach 
La ie re is shown in Fig. 188. The 
ib beach follows the general 

















Fig. 188. Cross-section of a beach. (Gil- boundary between water 
bert.) and land, though it does not 
conform to its minor irregularities (Figs. 174 and 187). The beach 
(or barrier) may deflect the lower courses of streams descending to it. 

In its deposition, shore drift assumes various forms. Where 
the bottom near shore has a very gentle inclination, the Incoming 
waves break some distance from the shore-line, and it is here that 
the most violent agitation occurs when the waves are strong. To — 
this line of breakers, material 


















YELL = is shifted from both direc- 
CII ———— 

“CU YpYeuU_ iI ; . 
LE EL tions. Accumulating here, it 


(Gilbert.) builds up a low ridge, called 

the barrier (Fig. 189). If it 
is built up above the surface of the water by storm-waves, it may 
shut in a lagoon behind it, and this may be filled ultimately by 
sediment washed down from the land. At one stage in the filling, 
the lagoon becomes a marsh (Fig. 191). 

The disposition of shore deposits depends largely on the currents 
at and near shore. If the coast-line is deeply indented, the littoral 
current usually tails to follow the re-entrants. In holding its course 
across the mouth of a small bay, the velocity of the shore-current is 
checked because it passes into deeper water. Deposition follows. 
The deposits are in a narrow belt which marks the course of the 
current, and the result is the construction of a ridge beneath the 
water. The current does not build the embankment up to the 


Fig. 189. Secticn of a barrier. 


SHORE DEPOSITS 185 


ra 





Fig. 190. An elevated barrier beach on the coast of California. 








Fig. ror. Sketch-map of a part of the New Jersey coast. The dotted belt at 
the east is the barrier, modified by the wind. The area marked by diagonal lines 
is the mainland; the intervening tract is marsh-land. The numbers show the depth 
of water in feet. Scale: 34 inch=1 mile. 


186 WORK OF THE OCEAN 





Fig. 192. A recurved spit, Dutch Point, Grand Traverse Bay, Lake Michigan. 
(U. S. Geol. Surv.) 
water-level; but when its surface approaches the level of effective 
agitation, the waves may build it up to the surface of the water, or 
even above it. So long as the end of such an embankment is free, 
it is a spit. The construction of a spit has been aptly compared 
to the construction of a railway embankment across a depression. 
The material is first carried out from the bordering upland (in this 
case the shallow water) and dumped where the slope to the de- 
pression (deep water) begins. The embankment thus begun is 
extended by carrying out new material, which is left at the end of 
the dump already made, as at the end of a railway grade. 

The spit is normally 
either straight or parallel 
with the general course of 
the shore-current, but since 
the littoral current is sub- 
ject to change with shifting 
winds, the spit may be- 
come curved or hooked 
(Fig. 192). 

If the spit is lengthened 
until it crosses, or nearly 
crosses, the bay, shutting it 
off from the open water it 
becomes a bar. Bars have 
shut in lakes, ponds, and 
lagoons aft numerous 


Fig. 193. Map of the head of Lake Superior. points both on the Atlan- 
(U. S. Geol. Surv.) tic and the Pacific coasts 





ea 


SHORE DEPOSITS 187 


(Pl. XV, Fig. 1 and Fig. 187). The same phenomena are to be 
seen along many lakes (Fig. 193). Bars may tie islands to the 
mainland (Pl. XV, Fig. 2). If the bay across which the bar is built 
receives abundant drainage from the land, the outflow from the 
bay may be sufficient to prevent the completion of the bar, for 
when the growth of the bar has narrowed the outlet of the 
bay sufficiently, the sediment brought to the end of the spit by 
the littoral current will be swept out by the current setting 
out from the bay. The completion of a bar may be interfered with 
also by tidal currents, even without land-drainage. The scour of 
the tides preserves deep entrances (inlets) to bays in some places, 
and maintains definite channels or “‘thorofares” in the lagoon 
marshes behind barriers and spits (Fig. 191). The sediment brought 
down from the land, as well as that washed in by tidal currents and 
waves, tends to fill up the lagoon behind a barrier, a spit, or a bar, 
converting it into land (Fig. ror). 

Since spits and bars are built only where there is shore-drift in 
transit, they are always built out from a beach or barrier. The 
distal end of the bar also may join a beach or barrier. Traced 
back to its source, the beach from which a spit leads is in many cases 
found to end at the cliff from which the material of the beach and 
spit were derived. 

The off-shore movements of shore-waters may leave the sediment 
of the shore in the form of a wave-built terrace, which is really a 
seaward extension of the beach. A wave-built terrace borders 
many wave-cut terraces along their seaward margins (Fig. 181). 
Terrace-cutting and terrace-building are both involved in the devel- 
opment of continental shelves. 

Beach ridges, spits, bars, etc., like sea-cliffs and wave-cut ter- 
races, are preserved for a time after the relative levels of sea and 
land have changed. If the shore has risen, relatively or absolutely, 
these features are evidence of the change. If shore features are 
submerged instead of elevated, they furnish less accessible though 
not less real evidence of the change. Similar features about lakes 
have a like significance, but there it is demonstrable, in many 
cases, that it is the water rather than the land which has changed 
level. 

Shore-deposition and coastal configuration. The tendency of 
shore-deposition is to cut off bays and to straighten and simplify 
the shore-lines. This is abundantly illustrated along the Atlantic 


188 WORK OF THE OCEAN 


and Gulf coasts of the United States (Figs. 174, 187, and Pl. XV, 
Fig. 1). It is to be noted, however, that in the simplification of 
the shore-line through deposition, the initial stages may result 
in great irregularity (Figs. 187 and 101). 


Ocean-currents 


Ocean-currents are due primarily to winds. As agents of ero- 
sion, they are not of great importance. Currents which reach the 
bottom are comparable, in their effects, to rivers of the same ve- 
locity and volume; but most ocean-currents do not touch bottom, and 
therefore do not erode it. Only where they flow through narrow 
and shallow passageways is their abrasive work considerable. Thus 
the Gulf Stream has a velocity of four or five miles per hour where 
it issues from the Gulf, and its shallow and narrow channel is cur- 
rent-swept. Other illustrations of the erosive power of currents 
have been noted near Gibraltar in water 500 fathoms deep, and 
between the Canary Islands at depths of tooo fathoms. In spite 
of such examples, it yet remains true that ocean-currents are on 
the whole but feeble agents of erosion. They are scarcely more 
important in transporting, for they carry little except that which 
they erode, if the life which lives in them is disregarded. Currents 
which do not touch bottom roll no sediment, and carry only what is 
held in suspension. A river’s power of transporting sediment in 
suspension is due largely to cross-currents occasioned by the un- 
evenness of its resistant bottom. If a particle of mud suspended in 
a river drops to the bottom, as it frequently does, it may be picked 
up again and carried forward. If, on the other hand, a particle 
suspended in an ocean-current once escapes the moving water by 
settling through it, the current which does not drag bottom has no 
chance to pick it up again. Very fine sediment may be carried by 
an ocean-current far from the point where it was acquired, but cur- 
rents which do not touch bottom are rarely strong enough to carry 
any but the finest material. 

How readily particles of extreme fineness may be kept in sus- 
pension, and how little agitation is necessary to keep them from 
sinking, is shown both by experiment and observation. Experi- 
ment has shown that fine particles of clay require days to settle a 
foot in still water, and the Challenger found fine sediment derived 
from the land 400 miles from the coast of Africa. Sediment settles 
more readily in salt water than in fresh, despite the fact that the 


SHALLOW-WATER DEPOSITS 189 


former is heavier. This is presumably because the salt diminishes 
the cohesion of water. 


DEPOSITS ON THE OCEAN-BED 


The deposits on the bed of the ocean may be divided into two 
classes'— shallow-water deposits, made in water less than about 100 
fathoms deep, and deep-sea deposits, laid down in water of greater 
depth. ‘The selection of the 1oo-fathom line as the dividing depth 
is less arbitrary than it seems, for passing outward from the shore, 
it is at about this depth that the bottom ceases to be commonly dis- 
turbed by the action of currents and waves; that sunlight and 
vegetable life cease to be important at the bottom; and that the 
coarser sediments which predominate along shore give place to 
muds and oozes. Furthermore, the 1too-fathom line (or some line 
near it) is an important one in the physical relief of the globe, for 
it appears to mark, approximately, the junction of continental 
plateaus and ocean-basins. Because the latter are a little overfull, 
the water runs over their rims, covering about 10,000,000 square 
miles of the borders of the continental protuberances. 

Aside from the deposits made by organisms, shallow-water 
deposits are divisible into two groups — (1) those immediately 
along the shore, the /ittoral deposits, and (2) those between the lit- 
toral zone and the too-fathom line. Both are terrigenous chiefly, 
though chemical and organic deposits occur in both. The deep-sea 
deposits likewise are divisible into two principal groups, (1) the 
terrigenous deposits near the land, and (2) the pelagic deposits, made 
chiefly of the remains of pelagic organisms, and the decomposed 
products of such other materials as reach the deep sea. 


Shallow-water Deposits 


Littoral deposits. The littoral zone is often defined as the zone 
between high- and low-water marks, but in common speech, the 
very shallow water a little farther from the coast-line is generally in- 
cluded. It is the zone in which sand and coarser materials accu- 
mulate, though muds are met with occasionally in sheltered estu- 
aries. Generally speaking, the nature of these deposits is deter- 
mined by the character of the adjoining lands and the local organ- 
isms. The heavier materials brought down by rivers or worn from 
the shore by waves are here spread out by waves and shore-currents, 

‘Murray, Challenger Report, Deep Sea Deposits, pp. 184, 185. 


190 WORK OF THE OCEAN 


Twice in twenty-four hours the littoral zone is covered by water, 
and twice parts of it are exposed to the direct rays of the sun or the 
cooling effects of the night. Physical conditions in general are here 
most varied. Still greater diversity is introduced by the fact that 
the zone is inhabited by both marine and terrestrial organisms, 
while the evaporation of the sea-water which flows over tidal 
marshes and lagoons leads to the formation of saline deposits. 
The length of the coast-lines of the world is some 125,000 miles 
(about 200,000 kilometers), so that the zone of littoral deposits, 
though narrow, covers a very considerable area. 

Extra-littoral deposits. These deposits are made between the 
littoral zone and the 100-fathom line, and cover an area of nearly 
10,000,000 square miles. Their composition is much the same as 
that of the littoral deposits except that they are finer. At their 
lower limit they pass insensibly into the fine deposits of the deep sea. 
Coarse materials, such as gravel and sand, prevail, though in 
depressions and inclosed basins, and out toward the oceanward 
edge of the zone, muddy deposits are found. Some of the deposits 
are composed wholly of inorganic debris, but organic remains are 
mingled freely with others. The mechanical effects of tides, cur- 
rents, and waves are everywhere present, but become less and less 
well marked as the 1oo-fathom line is approached. The forms of 














= 





bts, pis 
nn oe eT 
So ee 
. 
== 


Fig. 194. Diagram showing the interwedging of gravel, sand, and mud beds. 





vegetable and animal life are numerous, though the former decrease 
as depths which make the sunlight feeble are approached. 

No definite line marks the seaward terminus of the coarse 
detritus, since coarse material is carried farther out when the 
waves run high (and the undertow is strong) than when they are 
feeble. In calm weather fine sediment may be deposited where 
coarse was laid down in the preceding storm, to be covered in turn 
by deposits of a different character. Thus gravel grades into sand, 
with more or less overlapping or interwedging, and sand grades 
into silt in the same way. This is diagrammatically illustrated by 


Fig. 194. 


SHALLOW-WATER DEPOSITS IQI 


Since coarse deposits may extend far out from land where the 
waves are strong and the water shallow, and since the zone of shallow 
water may be extended seaward by the aggradation of the bottom, 
shallow-water deposits may cover extensive areas. They may 





Fig. 195. Diagrams showing how shallow-water deposits may attain consider- 
able thickness by the shifting of the zone of deposition seaward. 


become deep at the same time, for as the outer border of the shallow- 
water zone is shifted seaward by aggradation, the vertical space 
to be filled becomes greater (compare a and 8, Fig. 195). Again, 
if the coast is sinking, new deposits of coarse material may be made 
on older ones. In this way, also, great thicknesses of sediment may 





oe 
pac 


Fig. 196. Ripple-marks. 


be accumulated, all parts of which were deposited in shallow water. 
The great thicknesses of some of the conglomerate beds of the past 
show how far this may go. 


192 | WORK OF THE OCEAN ~- 


Characteristics of shallow-water deposits. Clastic sediments 
laid down in shallow water have several distinctive characteristics. 





Fig. 197. Rill-marks resembling impressions 
of seaweeds. Beach at Noyes Point, R. I. 
(Walcott, U. S. Geol. Surv.) 


While they are coarse as 
a whole, they are char- 
acterized by many varia- 
tions in coarseness. The 
surfaces of successive beds 
are likely to be ripple- 
and rill-marked (Figs. 
196 and 197), and cross- 
bedding (Figs. 198 and 
199) iscommon. Clayey 
sediments deposited be- 
tween high and low water 
may be sun-cracked 
(Figs. 200 and 201), and 
the tracks of land ani- 
mals are in some cases 
preserved on their sur- 
faces. Shallow-water 
deposits may contain fos- 
sils of organisms which 
live in waters of slight 
depth. These character- 
istics differentiate sedi- 
mentary formations 
made in shallow water 
from those made in deep 
water, even after they 
have been converted into 


solid rock, and after the rock has emerged from the sea. Many of 


these characteristics are, how- 
ever, shared by deposits made 
by streams on land. Subaérial 
and lacustrine sediments are 
distinguishable from those 
made in the sea by their fossils, 
their distribution, etc. 
Shallow-water deposits 


have, on the whole, a rather Fig, 108. 





Cross-bedding. (Gilbert.) 


SHALLOW-WATER DEPOSITS 19% 


plane surface, though there are some notable departures from flat- 
ness. The steep slopes of the delta fronts and of wave-built terraces 
have already been noted (pp. 187, 191). Barriers may shut in de- 
pressions, and the disposition of sediment may be uneven, owing 
to shore and tidal currents. The result is that the surfaces of shal- 
low-water deposits are affected by low swells and shallow sags. 
The swells and sags may be elongate, circular, or irregular in outline. 
This topography is in some cases preserved on newly emerged lands. 

Chemical and organic deposits in shallow water. There is no 
sharp line-of distinction between the deposits classed as chemical 
and those classed as organic. ‘The latter are chemical in the broader 





Fig. 199. Cross-bedded sandstone, Dells of the Wisconsin. The strata are 
horizontal. The laminz within each stratum dip notably. (Atwood.) 


sense of the term, but as they are directly associated with life 
and arise from it, it is a matter of convenience to separate them. 
Chemical deposits made in shallow sea-water embrace (1) those 
due to evaporation, and (2) those due to chemical reactions between 
constituents so brought together that new and insoluble compounds 
are formed and precipitated. 

The chemical deposits made in the shallow water of the sea, 
or in bodies of shallow water isolated from the sea, are chiefly 
precipitates resulting from evaporation. All substances in solution 
are necessarily precipitated on complete evaporation; but since the 
sea-water is in general far from saturation, so far as all its leading 
salts are concerned, only a few are thrown down in quantity suff- 
cient to be of geologic importance where evaporation is incomplete. 


104 WORK OF THE OCEAN 


The principal deposits of this sort are calcium carbonate (limestone, 
CaCOs;) calcium sulphate (gypsum, CaSO.,2H:O), common salt 
(rock salt, NaCl), and magnesium salts, chiefly the chloride and 
sulphate. 

While there is more than ten times as much lime sulphate as 
lime carbonate in the ocean (p. 167), deposits of the carbonate 
(including shells, coral, etc.) have been very much greater than 
those of the sulphate. This is due to the following facts: (1) The 





Fig. 200. Sun-cracks; flat of the Missouri a few miles above Kansas City. The 
sun-cracks on shore deposits are not essentially different. (Calvin.) 


sulphate is much more soluble than the carbonate, (2) rivers bring 
much more carbonate than sulphate to the sea, and (3) marine 
plants and animals extract the carbonate from the water for their 
skeletons, shells, etc. The secretion of lime carbonate by organisms 
is not dependent on the saturation of the water, but is carried on 
when the amount in solution is very small. 

The chief deposits of lime carbonate have been made through 
the agency of plants and animals, in the form of shells, coral, bones, 
and other devices for supporting, housing, protecting, and arming 
themselves; but while it is agreed that the larger part of the lime 
carbonate deposited in the open sea is of organic origin, it is equally 
clear that in closed seas subject to concentration from evaporation, 
direct precipitation may take place. There is difference of opinion 
as to the quantitative importance of this last class of deposits. 


SHALLOW-WATER DEPOSITS 195 


Gypsum appears to be deposited in quantity only in the basins 
of arid regions where concentration reaches an advanced state. 
Since normal sea-water is far from being saturated with common salt, 
the latter is precipitated only in lagoons, in closed seas, or other 
situations favorable to great concentration. This is, as a rule, 
only in regions which are notably arid. It follows that deposits of 
salt usually signify highly arid conditions, and where they occur 





Fig. 201. Records of sun-cracks in sandstone. 
size. (Geikie.) 
over wide ranges in latitude and longitude, as in certain periods 
of the past, general aridity of climate is inferred. Where confined 
to limited areas, their climatic significance is less, for topographic 
conditions may determine local aridity. The total area where salt 


About three-eighths natural 


is now being precipitated is small, though on the whole the present 


is probably a rather arid period of the earth’s history. On the 
other hand, ancient deposits of salt preserved in the sedimentary 
strata show that the area of salt deposition has been much more 
considerable than now at one time and another in the earth’s 
history. The salt and gypsum deposits of the past seem, therefore, 


to tell an interesting tale of the climates of bygone days. 


196 WORK OF THE OCEAN 


The magnesium salts are among the last to be thrown down as 
sea-water is evaporated, and they most commonly take the form 
of sulphates and chlorides. The magnesium salts are among the 
last to be precipitated, not only because they are readily soluble, 
but because their quantity is small; yet in the original rock from 
which the sea-salts came, there is at least as much magnesium as 
sodium, while in the sea there is about five times as much sodium 
as magnesium. Just what becomes of all the magnesium brought to 
the sea-water is not well understood. In the older marine strata, 
dolomite, composed partly or wholly of the double carbonate of 
lime and magnesia (CaMg)CO;, abounds. This appears to have 
been formed by a gradual substitution of magnesium for calcium 
in calcium carbonate, but just how and when and why the substi- 
tution was effected is not fully known. One view is that dolo- 
mite was formed chiefly in basins not freely connected with the sea. 

The plants and animals of the sea secrete notable quantities of 
silica, but deposits of this sort are relatively more important in the 
deep sea, and will be mentioned in that connection. 

Something concerning the origin of limestone has already been 
given in the preceding paragraphs; but because of the importance 
of this rock, it may be added, by way of summary, that shallow seas 
free or nearly free from terrigenous sediment, and abounding in 
lime-secreting life, furnish the conditions for nearly pure deposits 
of limestone, and that most of the limestone within the areas of the 
present continents appears to have originated under such conditions. 
The common notion that limestone is normally a deep-water forma- 
tion is an error. Although limestones are formed in deep as well as 
in shallow waters, the more important classes of lime-secreting 
organisms are limited to the depths to which light penetrates. After 
being formed, limestones may lose many of their original character- 
istics, but enough usually remain to tell the story of their origin. 


Deep-sea Deposits 


Contrasted with shallow-water deposits. Deep-sea deposits 
cover the ocean-bottom below the 1too-fathom mark. Their area is 
about two-thirds of the earth’s surface. The characteristic deposits 
are muds, organic oozes, and clays, which, in their physical 
characteristics, are remarkably uniform. In regions of floating 
ice, some diversity is introduced from the varied nature of the ma- 
terials which it transports. 


DEEP-SEA DEPOSITS 197 


The slow accumulation of sediment on the deep-sea bottom, the 
absence of transportation there, and the nature and small size of 
the particles, all favor chemical reactions which result in the forma- 
tion of many new products, such as glauconite, phosphatic and man- 
ganic nodules, zeolites,! etc. The amount of matter arising from the 
decomposition and alteration of minerals and rocks increases, rela- 
tively, with increase of distance from the land. At the same time 
there is an increase (relative), in all moderate depths, of the remains 
of pelagic organisms. We thus pass insensibly from deep-sea 
deposits of a terrestrial origin (¢errigenous deposits) near the land, 
to pelagic deposits, ‘‘in which the remains of calcareous and siliceous 
organisms, clays, and other substances of secondary origin play the 
principal role.”’ ” 

The following table * shows the relations of the various groups 
of marine deposits. 

( Red clay 


Radiolarian ooze 3 baits ‘ige ge ER: 
a Lon 9078 ( water far removed 
Globigerina ooze f eee 
1. Deep-sea deposits beyond | Pteropod ooze } a ac tie 
OS 0 ar Blue mud 
Red mud 
ee 5 II. Terrigenous depos- 
Becerra its formed in deep 
[ and shallow water, 
2. Shallow-water deposits ave mostly close to 
between low-water mark en sa els, | land 
Bnd too fathoms... ... 2.2... ae | 
3. Littoral deposits between § Sands, gravels, 
high- and low-water marks / muds, etc. J 


In spite of this classification of Murray, the coral and volcanic 
muds cannot be regarded as terrigenous, and shells, coral, etc., are 
found abundantly in shallow-water deposits. It is to be noted 
that the pelagic deposits are partly organic and partly inorganic 
in origin. The latter may be of mechanical or chemical origin. 

Mechanical inorganic deposits. The mechanical deposits of the 
deep sea come from (r) the land by the ordinary processes of grada- 
tion, (2) volcanic vents, and (3) extra-terrestrial sources. The 
terrigenous materials which reach the deep-sea are, as a rule, only 

1“ The generic name for a group of hydrated double silicates in which the 
the principal bases are aluminum, and calcium or sodium.” 


* Murray, Challenger Rept., Deep Sea Deposits. 
3 Ibid., p. 186. 


198 WORK OF THE OCEAN 


the finest products of land decay, carried out by movements of 
water and by winds. They are not commonly recognized in the 
dredgings move than 200 miles from shore, but opposite the mouths 
of great rivers they extend much farther — 1,000 miles in the case 
of the Amazon. ‘They are especially abundant on the slopes of the 
continental shelves, where the blue, green, and red muds are asso- 
ciated with volcanic and coral muds. The color of these various 
muds depends in part at least on the changes they have undergone 
since their deposition. These depasits are analogous, in a general 
way, to certain shales, marls, etc., found on the continents. 

The occasional presence of coarse materials from the land in 
the deep-sea deposits must be looked upon as in some sense acci- 
dental. Pebbles, or even bowlders, entangled in the roots of 
floating trees, may be carried out into the ocean, and icebergs carry 
out bowlders and smaller fragments of rock. Of the identifiable 
inorganic materials in the pelagic depesits, those of volcanic origin 
are most abundant. Their distribution is essentially universal, 
though not uniform. Some of them are probably from submarine 
volcanoes. 

Deep-sea deposits contain many nodules and grains believed 
to be of extra-terrestrial origin. The dust of the countless meteors 
which enter the atmosphere daily settles on land and sea alike, and 
must enter into the sediment at the bottom of the iatter. It is 
probably no more abundant in deep water than in shallow, bet it is 
relatively more important, since there is little other sediment. The 
number of meteorites which enter the atmosphere daily has been 
estimated at from 15,000,000 to 20,000,000.! If, on the average, 
they weigh ten grains each, probably a rather high estimate, the 
total amount of extra-terrestrial matter reaching the earth yearly 
would be 5,000 to 7,000 tons, and something like three-fourths of 
this must, on the average, fall into the sea. But even at this rate 
it would take some fifty billion years to cover the sea-bottom with 
a layer one foot in thickness. 

Organic constituents of pelagic deposits. With increasing dis- 
tance from shore, and especially with increasing depth of water, 
sediments derived from pelagic life increase in relative importance. 
Some pelagic animals and plants secrete lime carbonate, while 
diatoms and radiolarians secrete silica. When the organisms die, 


1 Young’s Astronomy, p. 472. It is now believed that these figures are too 
small. 


DEEP-SEA DEPOSITS 199 


they sink to the bottom and their secretions are mingled with the 
volcanic and other materials which are universal over the sea-floor. 

Pelagic deposits of organic origin are named according to their 
characteristic constituents. Thus there are pleropod oozes, globi- 
gerina oozes, diatom oozes, radiolarian oozes, etc. Diatom ooze is 
an ooze in which the secretions of diatoms are abundant, and 
globigerina ooze is an ooze in which globigerina shells are abundant, 
though in many cases the diatom and globigerina shells, respectively, 
do not make up the bulk of the ooze. Between the various sorts of 
oozes there are all gradations, since pelagic life does not recognize 
boundary lines. 

It is a significant fact that with increasing depth the proportion 
of lime carbonate in the ooze decreases. Thus in tropical regions 
remote from land, where the depth is less than 600 fathoms, the 
carbonate of lime of the shells of pelagic organisms may constitute 
80% or 90% of a deposit. With the same surface conditions, but 
with increasing depth, the percentage of lime carbonate decreases, 
until at 2,000 fathoms it is less than 60%; at 2,400 fathoms, 30%, 
and at 2,600 fathoms, 10%. Beyond this depth there are usually 
no more than traces of carbonate of lime. The data at hand show 
that the percentage of lime carbonate falls off below 2,200 fathoms 
more rapidly than at lesser depths. Where the percentage of lime 
carbonate becomes very low, the calcareous oozes grade off into the 
red clay with which the sea-floor below 2,400 to 2,600 fathoms is 
covered. 

Chemical deposits. The chemical deposits of the deep sea are 
chiefly the alteration products of sediments which reach the sea- 
bottom by mechanical means. All sediment deposited in the sea 
undergoes more or less chemical change, but it is only when the 
change is very considerable that the product is referred to this class. 
Where sedimentation is rapid and the sediment coarse, the chemical 
change is relatively slight; but where the sedimentation is slow 
and the sediment fine, the chemical change is relatively great; for 
both the longer exposure to the sea-water and the greater propor- 
tion of surface exposed to attack favor change. The red clay 
already referred to belongs to this class of deposits. Jt contains 
much volcanic debris,! various concretions, bones of mammals, 
zeolitic crystals, and extra-terrestrial spherules, and doubtless the 


1 Murray, Challenger Report on Deep Sea Deposits, p. 337 et seq., and Buchan- 
an, Proc. Roy. Soc. Edin., Vol. XVIII, pp. 17-39. 


200 WORK OF THE OCEAN 


insoluble parts of the shells of pelagic life. The nodules and crystals 
are secondary products, the materials for which were derived from 
the decomposition of the sediments which gave rise to the clay. 
Eolian dust, or the materials derived from it by chemical alteration, 
is doubtless a constituent of the red clay. 

It is significant that deposits corresponding to those of the 
deep sea have not been identified with certainty among the rock 
formations of the land. If such deposits are absent from the land, 
as they seem to be, their absence must mean that the continents 
have never been beneath deep seas. That large parts of them have 
been beneath shallow sea-water is abundantly attested. 


Map work. See Plates CXXX-CLIV, Professional Paper 60, U. S. Geological 
Survey, and Laboratory Manual, The Interpretation of Topographic Maps, Exercise 
XV. 


CHAPTER VII 
LAKES 


Many of the phenomena of the ocean are repeated on a smaller 
scale in lakes. The waves of lakes and their attendant undertows 
and littoral currents are governed by the same laws and do the 
same sort of work as the corresponding movements of the ocean. 
Tides are insignificant; but slight oscillations of level, known as 
seiches, have been observed in many lakes. They are probably 
caused by sudden changes in atmospheric pressure. Currents 
corresponding to those of the ocean are slight or wanting in lakes, 
but since most lakes have inlets and outlets, their waters are in 
constant movement toward the latter. In most cases this move- 
ment is too slow to be noted readily, or to do effective work either 
in corrasion or transportation. ‘The work of ice is relatively more 
important in lakes than in the sea. 

Changes taking place in lakes. The processes in operation in 
lakes are easily observed and readily understood. (1) The waves 
wear the shores, and the material thus derived is transported, 
assorted, and deposited as in the sea, and all the topographic forms 
resulting from erosion or deposition along the seacoast are repro- 
duced on their appropriate scale in lakes. (2) Streams bear their 
burden of gravel, sand, and mud into lakes and leave it there. 
(3) The winds blow dust and sand into them, and in some places pile 
the sand up into dunes along their shores. (4) Animals of various 
sorts live in lakes, and their shells and bones give rise to deposits 
comparable to animal deposits in the sea. (5) Numerous plants 
grow in the shallow water about the borders of many ponds and 
lakes, and as they die, their substance accumulates on the bottom. 
(6) The outlets of lakes which have outlets are constantly lowered 
by the outflow. The lowering is generally slow if the rock is coher- 
ent, for the outflowing water is usually clear, and therefore inefficient 
in corrasive work. ‘These six processes (except the last, which does 
not apply to lakes without outlets) are essentially universal, and all 
conspire against the perpetuity of the lakes. (7) In lakes where the 


201 


202 LAKES 


temperature is low enough for ice to be formed, it crowds on the 
shores and develops phenomena peculiar to itself (Figs. 126-127). 
(8) In some lakes in arid regions, deposits are made by precipitation 
from solution. 

Several of these processes are filling the basins of lakes, and 
as sediment is deposited in a lake, a corresponding volume of water is 
forced out if the lake has an outlet. The sixth process also is antag- 
onistic to lakes. Given time enough, these processes must bring 
the history of any lake to an end. The lowering of the outlet alone 
will accomplish this result if the bottom of the basin is above base- 
level. Many lakes already have become extinct, either through the 
filling or draining of their basins, or through both combined. It does 
not follow, however, that lakes will ever cease to exist, for the causes 
which produce them may operate contemporaneously with those 
which tend to destroy lakes now in existence. 

Lacustrine deposits. Beds of sediment deposited in lakes are 
similar in kind, structure, and disposition, to beds of sediment laid 
down in the sea; but in lakes river-borne sediment is more com- 
monly concentrated into deltas, since waves, tides, and shore-cur- 
rents are less effective than in the sea. Even the limestone of the 
sea has its counterpart in some lakes. Some of it was made of the 
shells of fresh-water animals which throve where the in-wash of 
terrigenous sediment was slight, some of it from the calcareous secre- 
tions of plants,’ and some of it was precipitated from solution.’ 
While still soft, such deposits are called marl. Salt deposits also 
are made in some lakes, and iron-ore in some marshy ones. 

Extinct lakes. The former presence of lakes where none now 
exist is known in various ways. Ifa lake basin was filled, its former 
area is a flat, the material of which bears evidence of its origin in its 
composition, its structure, and in its fossils. Such a flat com- 
monly is so situated topographically that the basin would be repro- 
duced if the deposits were removed. To this general rule there 
are exceptions, as where a glacier formed one side of the basin when 
it was filled. If the lake was destroyed by the lowering of its out- 
let, or by the removal of some barrier such as glacier ice, or by 
desiccation, shore phenomena, such as beaches, terraces (Fig. 202), 
spits, etc., may exist, even though there is no well developed flat 

1C. A. Davis, Jour. of Geol., Vol. VIII and Vol. IX. 


2 Russell, Mono. XI, U. S. Geol. Surv., Chap. V; also Third Ann. uae pp. 
211-221. Gilbert, Mono. Li Uys. Wen, Surv., p. roy 


LAKES 203 


corresponding to the bed of the lake. In time, such features are 
destroyed by subaérial erosion, so that they are most distinct soon 
after a lake disappears. 

Many lakes, some of them large and many of them small, are 
known to have become extinct,! while many others are now in their 


NYY 


Wages 
Wes 


ae YL: Zi, Up A Xi aN 
: Zi: Op. ~® CHESS 











COIR < 
FP OUT 8 i ae 
net ae eS SS 


VIEW FROM THE EAS T 





Fig. 202. Shore terraces of extinct Lake Bonneville, Wellsville, Utah. (Thomp:- 
son and Holmes.) 


last stages, viz. marshes. Many others have been reduced in size. 
Such reductions are obvious where deltas are built into lakes. Thus 
the delta built by the Rhone into Lake Geneva is several miles in 
length, and has been lengthened nearly two miles since the time 
of the Roman occupation. The end of Seneca (N. Y.) Lake (PI. 
XVI) has been crowded northward some two miles by deposition 
at its head. Similar changes are common. 

Salt lakes. A few lakes, especially in arid or semi-arid regions, 
are salt, and others are ‘“‘bitter.”’ Beside common salt, most salt 
lakes contain magnesium chloride, and magnesium and calcium 
sulphates, as well as other mineral substances. Most “bitter”’ 
lakes contain sodium carbonate, as well as sodium chloride and 
sulphate, and some of them borax. The degrees of saltness and 
bitterness range up to saturation. The water of the Caspian Sea 
(lake) contains, on the average, less mineral matter than that of 


1 Gilbert, Lake Bonneville, Mono. I, U.S. Geol. Surv.; Russell, Lake Lahontan, 
Mono. XI, U. S. Geol. Surv.; and Mono Lake, Eighth Ann. Rept., U. S. Geol. 
Surv., Pt. I; Upham, Lake Agassiz, Mono. XXV, U. S. Geol. Surv.; Salisbury and 
Kiimmel, Lake Passaic, Rept. of the State Geologist of N. J.. 1893, and Jour. of 
Geol., Vol. III, pp. 533-560. 


204 LAKES 


the sea; that of Great Salt Lake contains about 18%; that of the 
Dead Sea, about 24%. 
Many salt lakes, such as Dead Sea and Great Salt Lake, are 





Fig. 203. Terraces on the shore of the ancient Lake Lahontan, north of Pyra- 
mid Lake, Nevada. (Fairbanks.) 


descended from lakes which were fresh, while others, like the Caspian 
Sea, are probably isolated portions of the ocean. Most lakes 
of the former class have become salt through a decrease in the 





Fig. 204. Tufa domes, Pyramid Lake, Nevada. (Fairbanks.) 


humidity of the region where they occur. The inflowing waters 
bring in small amounts of saline matter, and the water begins to 
be salty when the aridity is such that evaporation from the lake 


LAKES 205 


exceeds its inflow. Under these conditions concentration may go 
on to saturation. 

Deposits of salt and other mineral matters are now making in 
some salt lakes, and formations of the same sort have been made in 
the past. Buried beneath sediments of other sorts, beds of salt or 
other precipitates are preserved for ages. Lime carbonate has been 
precipitated in quantity from some extinct lakes (Fig. 204). 

Lakes which originate by the isolation of portions of the sea 
are salt at the outset. If inflow exceeds evaporation, they become 
less and less salty, and may become fresh ultimately; otherwise 
they remain salt. If evaporation exceeds inflow they diminish in 
size and their waters become more and more salt or bitter. 

Indirect effects of lakes. Lakes tend to modify the climate of 
the region where they occur, both by increasing its humidity and 
by decreasing its range of temperature. They act as reservoirs 
for surface-waters, and so tend to restrain floods and to promote 
regularity of stream flow. They purify the waters which enter 
them by allowing their sediments to settle, and so influence the 
work and the life of the waters below. 

_ Origin of lake basins.'. Lake basins arise in many ways, some 
of which have been pointed out. Most of them arise through 
processes of gradation. Some are formed by rivers (p. 114), some 
by waves and shore-currents (p. 186), and some by glacial erosion 
and deposition. Others are formed by volcanic action, as we shall 
see, and some by warpings of the earth’s surface. A few originate 
in other ways. 

1 Salisbury’s Physiography, Advanced Course, p. 303. 


CHAPTER VIII 


THE MOVEMENTS AND DEFORMATIONS OF THE EARTH’S 
BODY (DIASTROPHISM) 


The outer parts of the lithosphere are subject to a variety of 
movements, some rapid and some slow, some slight and some great, 
some limited to small areas, some affecting extensive tracts, and 
some involving the whole earth. For present purposes, they may 
be classed as (1) small and rapid, and (2) great and slow. Sudden 
movements of local masses, such as avalanches and landslides, are 
put in the first class. 


MINUTE AND RAPID (SEISMIC!) MOVEMENTS 


The crust of the earth is in a state of perpetual tremor. For 
the most part, these tremors are too slight to be sensible, though 
detected by delicate instruments. Some of them precede or follow 
earthquake vibrations, but more of them have no connection with 
violent movements. Many spring from the ordinary incidents of 
the surface, such as waves, waterfalls, winds, tides, the tread of 
animals, the rumble of traffic, and the blasting in mines. Move- 
ments due to such causes demonstrate the elastic nature of the crust, 
but are not considered here. 


Earthquakes * 


Earthquakes are tremors of appreciable violence springing from 
sources within the earth. The causes are various. The most 
common is probably the slipping of rock masses on each other in 
the process of faulting (Chapter X). To the same class belong 
movements due to slumping, which is superficial faulting. Tremors 

1 The science of earthquakes is Seismology. Earthquakes and other similar 
movements are seismic movements. The instruments which record seismic move- 
ments are seismographs, etc. 

2 Recent and instructive books on Earthquakes are Dutton’s Earthquakes; 
Hobbs’s Earthquakes. An Introduction to Seismic Geology; Milne’s Earthquakes | 
(4th ed.), and the same author’s Seismology; and Knott’s Physics of Earthquake 
Phenomena, 


200 


EARTHQUAKES 207 


attend many volcanic eruptions, and are attributable to the sudden 
fracture and displacement of rock by the movements of lava, or by 
the expansion due to heating. Quakes have also been attributed 
to the sudden generation or cooling of steam in underground con- 
duits, crevices, and caverns, and to the collapse of the roofs of 
subterranean caves. 

Points of origin; foci. It is probable that nearly all earthquakes 
start within ten miles of the surface, and most of them within five. 


£. tn 
— J 





Fig. 205. Diagram illustrating by closed curves the different rates of propaga- 
tion of seismic tremors from a focus F’, and, by lines normal to these, the changing 
directions of propagation of the wave-front. Propagation is least rapid toward 
the surface where rocks are least elastic. The paths of propagation curve upwards 
in approaching the surface. If the lines of emergence, as at E and E’, aré projected 
downward in straight lines to F’, the point of crossing will be below the true focus. 
The line at the top of the Fig. represents the surface of the earth. 


The older calculations which placed some of the foci much deeper, 
appear to be defective. 


The depth of the sources of disturbance is usually estimated by noting the 
directions in which bodies at the surface are thrust during an earthquake, plotting 
these directions, and projecting them backwards to their underground crossings 
(lines EF’ and E’ F’, Fig. 205). In the case illustrated by Fig. 205, this would 
place the focus at F’, This method gives only a rude first approximation to the 
location of the focus, which may be a point, a line, or a plane. The earthquake 
wave travels out from the focus with unequal velocity in various directions. This 
is because the rock varies in density, elasticity, temperature, etc. The aggregate 
_ effect of these variations is to make earthquake waves travel more slowly toward 
the surface than in other directions, and more and more slowly as the surface is 
approached. This is illustrated by Fig. 205, in which each closed curve connects 


208 MOVEMENTS AND DEFORMATIONS 


points reached by the wave at the same moment. The lines normal to these curves 
represent the directions in which the wave is propagated, in its various parts. The 
meeting point of these lines gives the true focus, F, which is much nearer the 
surface than F’. 


Amplitude of vibrations. From the disastrous effects of earth- 
quakes it might be inferred that the vibrations have large ampli- 


— 
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Phase of Vibrations: 


SSS 


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26 25 24 






Fig. 206. Seismogram of earthquake in Punjab, India, April 4, 1905, showing 
the actual amount of movement. (Montessus de Ballore.) 


tudes; but it is chiefly their suddenness that makes them effective. 
Except at their points of origin, most of them are only a fraction of a 
millimeter, and few exceed a few millimeters. It is the oscillation 
of the rock particles transmitting the vibrations that is here meant, 


- £ 


Fig. 207. A fissure on East Street, San Francisco, near the water front, in 
“made ground.” (Lindgren, U. S. Geol. Surv.) 


EARTHQUAKES 209 


not the movement of objects on the surface, which may be much 
greater. A sudden shock with an amplitude of 5 or 6 millimeters 


is sufficient to shatter a 
chimney. 

Destructive effects. 
The disastrous effects of 
earthquake shocks result 
from (1) the suddenness 
and strength of rather 
small vibrations of earth- 
matter, and from (2) the 
freedom of motion of the 
bodies affected. The deep- 
er rocks probably transmit 
seismic vibrations without 
appreciable disruptive 
effect; but bodies at the 





Fig. 208. Track of electric railway, between 
South San Francisco and San Eexno Point. 
(Photo. by Moran.) 


surface are broken, overturned, and displaced. The tap of a ham- 
mer sends an almost imperceptible vibration along the floer; but 
this vibration would throw a glass ball beneath which it runs con- 


siderably above the floor. 





Similarly the minute seismic vibrations 


Fig. 209. Great sea-wave on the coast of Ceylon. (Sieberg.) 


210 MOVEMENTS AND DEFORMATIONS 


travel miles from their origin through continuous substance with 
little result, and yet may then hurl a loose or unstable body to de- 
struction. Earthquake waves striking the sea-border may thrust 
the waters off shore, and the return wave may overwhelm the coast 
(Fig. 209). Sea-waves doubtless arise also from sudden seismic 


vibrations on the sea-bottom. 
Rate of propagation. The progress of a seismic wave varies 


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The two epicentral tracts are indicated by arbitrary 
isoseismal curves; the heavy line being the indez. 


SCALE OF MILES 
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BORMAY ENG. CO., N.Y. 

































































Epicentral tracts of the Charleston earthquake, with isoseismai 


Fig. 210. 
(Dutton, U. S. Geol. Surv.) 


lines (lines of equal disturbance). 


EARTHQUAKES 211 


appreciably. The violent vibrations on the surface near the 
epicentrum (point above the focus) are the most irregular, and 
strong vibrations generally have greater speed than weak ones. 
Vibrations propagated to great distances through and around the 
earth are less irregular in rate. Those which follow the surface 





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Fig. 211. ‘Coseismal lines (lines connecting places feeling the shock at the 


same time) for each minute; Herzogenrath (Germany) earthquake of October 22, 
1873. (Lasaulx.) 








travel about 1.85 miles (3 km.) per second. Those which go through 
the earth travel more rapidly, at rates ranging from about 3.9 miles 
to about 5.7 miles per second. 

Distribution. Over large parts of the globe, severe earthquakes 
are rare, but in certain regions they are, unfortunately, frequent. 
Earthquakes are likely to be rather frequent where geologic changes 
are in rapid progress, as along belts of young mountains where 
stresses are not yet adjusted, or at the mouths of great streams where 
deltas are accumulating, or about volcanoes where temperatures 
and strains are changing, or on the great slopes, particularly the 
submarine slopes, where adjustments to inequalities of stress are 
in progress. Not a few, however, occur where the special occasion 
is not obvious. 

Geologic Effects of Earthquakes 

Geologically, earthquakes are of less importance than many 
gentler movements. Disastrous as they are to human affairs, they 
leave few distinct marks which are more than temporary. 


212 MOVEMENTS AND DEFORMATIONS 








































































































































































































































































































































































































































































































































































































































































































Fig. 212. Map showing in black the principal earthquake regions of the Old 
World. (Montessus de Ballore.) 


Surface changes. During the passage of notable earthquake 
waves the solid rock may be fractured, though the fractures are 
rarely observable at the surface where the rock is covered by deep 
soil. In a few instances, surface-rock has been seen to be thorough- 
ly shattered by the passage of an earthquake, as in the Concepcion 
earthquake of 1835. Joints which were closed before, may be 
opened during an earthquake. Thus in northern Arizona, not far 
from Canyon Diablo, there is a crevice traceable for a considerable 
distance which is said to have been opened during an earthquake 
(Fig. 214). Locally, it gapes several feet. During an earthquake 
which shook the South Island of New Zealand in 1848, “‘a fissure 


EARTHQUAKES 213 


























































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































Fig. 213. Map showing the principal earthquake regions of the New World. 
(Montessus de Ballore.) 


was formed averaging 18 inches in width, and traceable for a dis- 
tance of 60 miles, parallel to the axis of the adjacent mountain 
chain.””' The development of fractures or the opening of joints 
is in some cases accompanied by faulting. This was the case in 
Japan during the earthquakes of October 28, 1891, when the surface 
on one side of a fissure, which could be traced for 40 miles, sank 2 to 
20 feet (Fig. 215). There was also notable horizontal displacement, 
the east wall of the fissure being thrust locally as much as 13 feet 
to the north. 

Circular surface openings or basins are developed in some cases 


1 Geikie, Textbook of Geology, 4th ed., p. 372. 


214 MOVEMENTS AND DEFORMATIONS 


during earthquakes, especially where the surface material is inco- 
herent. This was the case during the Charleston earthquake of 
1886,' and similar effects have been noted elsewhere. The basins 
are commonly sup- 
posed to be the 
result of the col- 
lapse of caverns, 
or other subterra- 
nean openings, the 
collapse causing 
“nll! the forcible ejec- 
tion of water in 
some cases. Sand 
cones and crater- 
lets are developed 
Si aR EO ee eS : by some earth- 

ae quakes (Fig. 216). 
During the Cali- 
fornia earthquake of 1906, the ground was much broken along 
the line of the fault which caused the shock (Fig. 217). Earth- 
quakes may dislodge masses of rock in unstable positions, as on 
slopes or cliffs, causing slumps and landslides. 


Ht 
an 
tt 





Fig. 214. Fissure produced by earthquake. Arizona. 





Fig. 215. Fault in Japan, 1891. (Koto.) 
1 Dutton, Ninth Ann. Rept., U. S. Geol. Surv., pp. 209-528. 


Ne ena. 


EARTHQUAKES | 218 


Effects on drainage. The fracturing of the rock may interfere 
with the movement of ground-water. After new cracks are de: 
veloped or old ones opened or closed, the movement of ground- 
water adapts itself | 
to the new condi- 
tions. It follows 
that a spring may 
cease to flow after 
an earthquake, 
while new ones 
break out where 
there had been 
none before. The 
character of the 
water of springs is 
in some cases 
changed, presum- 
ably because it 
comes from differ- 
ent sources after the earthquake. Joints may be so widened as to 
intercept rivulets. 
Where faults ac- 
company earth- 
quakes, they may 
occasion ponds or 
falls where they 
cross streams. 

Effects on stand- 
ing water. Some of 
the most destruc- 
tive effects of earth- 
quakes are felt 
along shores. The 
great sea-waves of 
the Lisbon earth- 
quake (1775) and 
of the earthquake 
on the coast’ of 
Ecuador and Peru 





Fig. 216. Sand cones and craterlets observed after an 
earthquake in Greece, in 1861. (Schmidt.) 





Fig. 217. Characteristic surface appearance of the 
California fault line, south end of Tomales Bay. - 
_ (Photo. by Newsom.) | in 1868, were very 


216 MOVEMENTS AND DEFORMATIONS 


destructive. Such waves have been known to advance on the land 
as walls of water 60 feet high. They are most destructive on low 
coasts where the water sweeps over great areas of land. Great 
loss of life may be caused by such waves. 

Earthquake shocks are remarkably destructive to the life of 
lakes and seas. Thus during the Indian earthquake of 1897, “‘ fishes 
were killed in myriads as by the explosion of a dynamite cartridge. 

. and for days after the earthquake the river (Sumesari) was 
choked with thousands of dead fish. . . . and two floating car- 
casses of Gangetic dolphins were seen which had been killed by 
the shock.’! This wholesale destruction of life is of interest since 
the surfaces of layers of rock, even of great age, are in some cases 
covered with fossils in such numbers as to indicate that the ani- 
mals were killed suddenly and in great numbers, and their bodies 
quickly buried. It has been suggested that such rock surfaces 
may perhaps record ancient earthquake shocks. 

Changes of level. Permanent changes of level accompany 
some earthquakes. Thus after the earthquake of 1822 ‘‘the coast 
of Chili for a long distance was said to have risen 3 or 4 feet.” 
Similar results have occurred on the same coast at other times, and 
on other coasts at various times. Depression of the surface is 
perhaps even more common than elevation. Thus on the coast of 
India, all except the higher parts of an area 60 square miles in extent 
were sunk below the sea during an earthquake in 1762. Wide- 
spread depression in the vicinity of the Mississippi in Missouri, 
Arkansas, Kentucky, and Tennessee accompanied the earthquakes 
of 1811 and 1812. Some of the depressed areas were converted 
into marshes, while others became the sites of permanent lakes. 
Reelfoot Lake, mainly in Tennessee, is an example. Change of 
level is involved in much of the faulting which goes with earth- 
quakes. 

Changes of level are not confined to the land. Where earth- 
quake disturbances affect the sea-bottom in regions of telegraph 
cables, the cables may be broken. In some such cases notable 
changes have been discovered when the cables were repaired. In 
one instance (1873) the repairing vessel off the coast of Greece? found 
about 2,000 feet of water where about 1,400 feet existed when the 
cable was laid. In another instance (1878) the bottom was so 


1 Oldham, loc. cit., p. 80. 
2 Forster, Seismology. Summarized in Am. Geol., Vol. III, 1889, p. 182. 


SECULAR MOVEMENTS 


217 
“irregular and uneven for a distance of about two miles, that a 
detour was made and the cable lengthened by five or six miles.’ 


In still another case (1885) the repairing vessel found a ‘‘ difference 


“b/d EY, aey, 


we 
7 


\ 





Fig. 218. Line of the fault where movement took place during the California 
earthquake of 1906. 


(U.S. Geol. Surv.) 


of 1,500 feet between the bow and stern soundings.” 


These records, 
if correct, point to sea-bottom faulting on a large scale. 


SECULAR MOVEMENTS 


The minute and momentary oscillations of earthquakes are very 


unlike the slow movements of continents or ocean basins, or even 
the slow wrinkling of mountain folds. 


Rivers may wear down 
their channels across a mountain range as fast as it rises across 


218 MOVEMENTS AND DEFORMATIONS 


their courses, and the movements of continents are yet slowe?; but 
far apart as these contrasted movements are, they are doubtless 
associated in cause. Many earthquake shocks are but incidents in 
the formation of mountains or in the movements of continents. 
Great movements may be classified variously, as (1) continent- 
making, (2) plateau-forming and (3) mountain-folding; as (1) gen- 
eral (epeirogenic) and (2) concentrated (orogenic); as (1) vertical 
and (2) horizontal; and dynamically, as (1) thrust and (2) stretching 
movements. These distinctions are analytical conveniences, but 
the various types of movement are not exclusive of one another, 
for continental movements may involve mountain-making, verti- 
cal movements involve horizontal movements in most cases, and 
stretching usually attends the outward bends of thrust folds. 
Present movements. Observations on seacoasts show that 
some shores are rising slowly and some sinking slowly, relative to 
sea level. It is not certain what these movements are, relative to 
the center of the earth. Theoretically all parts of the coast may be 
sinking, some faster than others, while the ocean-surface goes down 
at an intermediate rate; or all parts may be rising, but at different 
rates; or, again some lands may be actually rising relative to the 
center of the earth, and others sinking, while the ocean-level has 
an intermediate movement or none at all. We are accustomed 
to take the sea-level as a standard, as though it were stationary, 
which is probably not the fact. A general shrinkage of the earth 
is probably going on, carrying down the surface of both land and sea. 
It is possible that the shrinkage is so great that many of the upward 
warpings and foldings do not equal it. If this is true, most move- 
ments are really toward the earth’s center. There is a popular 
predilection for regarding earth movements generally as ‘“‘up- 
heavals,”’ and for regarding the rigid land as moving and the 
mobile sea-level as fixed. In reality, the sea is an extremely adap- 
tive body which settles freely into the depressions of the lithosphere, 
and is shifted with every warping of the latter. Whatever change 
affects the capacity of the sea basins affects the sea-level. If the 
basins are increased, the sea settles deeper into them; if they are 
decreased, the sea spreads out more widely over their borders. The 
one thing that gives a measure of stability to the sea-level is the 
fact that all the great basins are connected, and so an average is 
maintained. For this reason the sea-level is the most convenient 
basis of reference, and has become the accepted datum-plane, not- 


SECULAR MOVEMENTS 219 


withstanding its instability and its complete subordination to the 
lithosphere. If there were some available mode of measuring the 
distance of surface points from the center of the earth, it would 
reveal much that is now uncertain respecting the real movements 
of the surface. 

Periodic and aperiodic movements. The existence of land de- 
pends on protuberances of the surface of the lithosphere. If the 
lithosphere were perfectly spheroidal, water would cover it every- 
where to a depth of nearly two miles.. To maintain the existence of 
land the protuberances must be renewed from time to time; otherwise 
the land would in time be degraded to the lowest depths of wave 
action. Such renewal has been brought about again and again in 
geologic history. With every movement which restored the pro- 
tuberances, the oceans seem to have withdrawn more completely 
within the basins, while the continents have stood forth more | 
prominently until worn down again. ‘This renewal of protuberances 
appears to have been periodic in its great features, with long inter- 
vals between. In these intervals, the land was worn down by 
rivers, waves, etc., and the sea encroached upon the lower parts of 
the continents,— the continental shelves. Before complete sub- 
mergence was effected, renewed deformations checked the progress 
of submergence and rejuvenated the continents. 

Beside the great periodic movements, minor warpings or oscil- 
lations of the surface have been in almost constant progress. Some 
of these are probably incidental to the larger movements, but others 
probably are due to local causes. 


Minor Movements 


Gentle movements seem to have affected nearly every portion 
of the surface of the lithosphere at nearly all stages of its history. 
They have had much to do with the particular places and forms 
of deposits. Slow sinkings of sea-borders have permitted deposition 
to go on in shallow water for long periods without building the bot- 
tom up into land, and slow relative swellings of land tracts have 
renewed the sources of sediments for such deposits. Such. move- 
tents shift shore lines, and with them the areas of erosion and 
deposition. These movements may have amounted to a few inches, 
or a tew feet per century. In some cases they appear to have been 
reciprocal, one area being bowed up while another near by is bowed 
down. How far these are merely local or regional, due to loading, 


220 MOVEMENTS AND DEFORMATIONS 


unloading, changes of temperature, or other local causes, and 
how far they are the milder phases of great movements or incidental 
to them, it is difficult to decide. 


The Great Periodic Movements 


1. Mountain-folding. Along certain tracts, the shell of the 
earth has wrinkled, forming folded mountains. ‘The shell so folded, 
judged from the nature of the folds, seems to be no more than a 
few miles thick. The forces that caused the folding took the form 
of lateral thrusts. The folds themselves were usually lifted, show- 
ing that there was an upward component to the horizontal thrust, 
but the horizontal component was the dominant one. Some folds 
are nearly upright and symmetrical, and some inclined and asym- 
metrical, as illustrated in Chapter X. Where the folds lean, it is 
commonly inferred that the active thrust was from the side of the 
gentler slope, pushing the fold over toward the resisting side; but 
this is not a safe inference in all cases, for the original attitude of 
the beds has much to do with the way they yeild. Most systems of 
folded mountains embrace a series of roughly parallel folds, the 
whole forming an anticlinorium (Fig. 219). 

Distribution of folded mountains. The location of folded 
mountains is near the borders of continents in so many cases that 
the relation is probably significant, but there are folded mountains 
far from coasts, as the 
Urals, the mountains 
of Central Europe and 
of Central Asia. | 

Folding move-- 
ments seem to have 

Fig. 219. Anticlinorium: diagrammatic. (Van been very common in 
Hise, U. S. Geol. Surv.) the early ages. The 
Archean rocks(Chapter 


XIII) are almost universally crumpled, and in many places in the 
most intricate fashion, and the Proterozoic formations are much 
folded. After the inauguration of the Paleozoic era, folding appears 
to have taken place chiefly at long intervals, and for any given 
period to have been concentrated along certain tracts. The Ap- 
palachian system is an example. 

2. Plateau-forming movements. An important phase of mas- 
sive movement was the relative settling or raising of great blocks or 





SECULAR MOVEMENTS 221 


segments of the earth, as though by vertical, rather than horizontal 
forces. The great plateaus are examples of one phase of this action, 
and perhaps the great ‘‘deeps” of the ocean bottom, and some of 
the basins or troughs (Graben) of the continents, are examples of 
the other. Most plateaus are made up of numerous blocks which 
have been moved by different amounts. At the surface, these 
blocks are separated by fault-planes, but below, some of the faults 
pass into flexures. Plateau-forming movements are to be compared 
NEVADA | 
aN , oe igen oa na SALT 


ete ET AN a ge he =. ‘ ~- 
z. i 
= = ot ‘ se a >, VALLEY 












Fig. 220. Ranges of the Great Basin. Length of section, 120 miles. (Gilbert.) 


with continent-forming movements rather than with mountain-fold- 
ings, differing from the former chiefly in magnitude. Plateaus 
may be regarded as parts of a continental mass that have suffered 
additional movement. Plateaus standin some such relation to con- 
tinents as one fault block of a plateau does to the whole plateau. 
3. Continent-forming movements. These are widespread 
movements affecting large masses of the body of the earth, if not its 
whole outer portion. Two or more continents may be affected by 
similar movements at the same time, and it is the view of many 
geologists that all continents are affected simultaneously by move- 
ments of a like kind, resulting in emergence or submergence, while 
the ocean basins are affected by movements of the opposite phase. 
These movements are regarded as reciprocal, and parts of a world- 
wide adjustment. While well supported both by observation and 
theory, this view is not universally accepted. Movements of this 
class seem to have started early in the history of the earth, and to 
have been renewed from time to time, rejuvenating the continents 
and deepening the ocean basins. Under the view that the earth is 
essentially solid throughout, these movements are regarded as 
extending down to great depths, while mountain folding is regarded 
as but the wrinkling of the earth’s skin to fit its changed body. 
Downward movements are regarded as the primary ones, and 
horizontal movements as a necessary result of them. The under- 
lying cause of movement is believed to be shrinkage due to an in- 
crease in the density of the earth, caused by gravity and by molec- 
ular and sub-molecular attractions. Cooling is probably a lesser 
cause of shrinkage. The master movement is thought to be the 


3232 MOVEMENTS AND DEFORMATIONS 


sinking of the ocean basins, whose specific gravity is greater than 
that of the continents. If the ocean basins and the continents, 
respectively, be regarded as the surfaces of great segments of the 
earth all of which are crowding toward the center, the stronger and 
heavier segments may be conceived to take precedence, squeezing 
the weaker and lighter ones between them. The consequent swell- 
ing up of the lighter segments accounts for the relative protrusion 
of the continents. 

The area of the depressed segments is almost exactly twice that 
of the protruding ones, if we count the 10,000,000 square miles of 
the continental shelves as parts of the latter. In millions of square 
miles, the depressed segments are approximately as follows: the 
Pacific 60, the Indian 27, the South Atlantic 24, the North Atlantic 
14, leaving 8 for minor depressions. The elevated segments are the 
Eurasian 24, the African 12, the North American to, and the South 
American 9, leaving 10 for the minor blocks. 

The downward movement of the larger segments and the 
crowding of the smaller and lighter segments between them involves 
deformation of the latter. The movements that spring from the 
deeper crowding affect the continental protuberances generally, or 
at least broadly, while the crowding of the more superficial parts 
affects the lands more locally. According to this view, it is obvious 
there should be special bowings on the borders of the continental 
segments, and this tallies with the archings common on borders 
of the continents, even where there is no folding. The shell of the 
earth is free at the surface, and as a result, folding and faulting are 
the modes of easiest accommodation there: while the deeper ee 
under great pressure, must be deformed throughout. 

The periodicity of the movements is assigned to the rigidity of 
the thick, massive segments which must be deformed to effect 
readjustment after shrinkage. Because of this rigidity, stresses 
accumulate for a time until they are equal to the resistance opposing 
them. A further increase of the stresses then causes yielding and 
readjustment. When masses under stress once begin to yield in 

the direction of their free surfaces, their attitudes for resistance be- 
come less favorable, and hence the yielding continues until the stress 
is eased. After this another period is required for stresses to ac- 
cumulate sufficient to produce another general deformation. Mean- 
time the minor stresses that may remain, or may be produced by 
the great deformations, tend to ease themselves and thus give rise 


SECULAR MOVEMENTS 222 


to minor movements (p. 219). Other minor movements are doubt- 
less due to local causes. 

Extent of the movements. Between the highest elevation of 
the land and the lowest depth of ocean, there is a vertical range 
of nearly twelve miles. From the Tibetan plateau, where a con- 
siderable area exceeds three miles in height, to the Tuscarora deep, 
where a large tract exceeds five miles in depth, the range is eight 
miles. This represents fairly the vertical range of differential 
movement of large areas, though not areas of continental size. 
The average height of the continents is about three miles above the 
average bottom of the oceans, and this may be taken roughly as the 
differential vertical movement of the segments of continental 
dimensions. 

If the protruding portions of the lithosphere were graded down 
and the basins graded up to a common level, this level would lie 
about 9,000 feet below the surface of the sea. Referred to this 
datum plane, the continents have been squeezed up relatively about 
two miles, and the basins have sunk about one mile. The total down- 
ward movement, representing the total shrinkage due to increase of 
density, is quite unknown, but from theoretical considerations, it 
would appear to be far greater than the differential movement. This 
would mean that all segments have probably moved toward the 
center, the basin segments about three miles more than the con- 
tinental. 

The extent of the /ateral movements of the shell has a peculiar 
interest, for it has a theoretical bearing on the extent of the down- 
ward movements. Every mile of descent of the crust represents 
more than 6 miles (6.28=27) shortening of the circumference. If 
the vertical movements were limited to the relative ones just named, 
the mile of descent of the ocean basins would give but little more 
than 6 miles excess of circumference for lateral thrust and the 
crumpling of the shell. How far does this go in explaining moun- 
tain folds? The shortening represented by the folds of the Alps 
has been estimated at 74 miles;! the shortening for the Appalachians 
in Pennsylvania, not including the crystalline belt on the east, 
at 16 miles;? that of the Laramide Range in British America at 25 
miles.* 

1 Heim, Mechanismus der Gebirgsbildung, p. 213. 


2 Chamberlin, R. T., Jour. Geol., vol. 18, p. 255, 1g10. 
3’ McConnel, Geol, Surv. of Canada, p. 33 D, 1886. 


224 MOVEMENTS AND DEFORMATIONS 


These estimates cannot be taken as measurements, but they are 
sufficiently close approximations to make it clear that the shortening 
of the shell involved in mountain folding is large. These estimates 
represent only that shortening of the circumference effected at cer- 
tain times and places; the whole shortening of a circumference 
involves the shortening implied by all the transverse folds on a 
given great circle. Usually a great circle does not cross more than 


one or two strongly folded tracts of the same age, from which it is — 


inferred that the shortening on each great circle at any one time 
was concentrated largely in a few tracts running at large angles to 
each other. If the folding of one of the main mountain ranges be 
doubled, it may perhaps represent roughly the shortening for the 
circle at right angles to it, for its own period of folding. If one is 
disposed to minimize the amount of folding, the estimate of the 
shortening may perhaps be put at 50 miles on a circumference, 
for each of the great mountain-making periods; or, if disposed to 
make the estimate large, the shortening may be put at 100 miles. 
For the whole shortening since the beginning of the Paleozoic era, 
perhaps twice these amounts might suffice. Assuming the cir- 
cumferential shortening to have been 50 miles during a given great 
mountain-folding period, the appropriate radial shortening is 8 
miles. For the more generous estimate of roo miles, it is 16 miles. 
If these estimates are doubled for the whole of the Paleozoic and 
later eras, the radial shortenings are 16 and 32 miles, respectively. 
If these or similar figures are correct, it is clear that the surface of 
the earth has sunk toward the center by an average amount greater 
than that of the highest mountains above mean sphere level, since 
the beginning of the Paleozoic era. The shortening for earlier eras 
can hardly be estimated from present data. 


Causes of Secular Movements 


The volume of the earth is affected by two sets of forces, acting in opposition 
to one another, (1) the concentrating forces, consisting of (a) gravity and (b) molec- 
ular and sub-molecular attractions, and (2) the forces which resist concentration 
consisting of (a) heat and (b) molecular and sub-molecular resistances. 

1. The centripetal forces. The best known of the concentrating forces is 
gravity, which tends to bring all parts of the earth as near the center as possible, 
the heavier beneath the lighter. The gravitative force of the earth causes a 
pressure of about 3,000,000 atmospheres at its center, and lesser pressures at lesser 
depths. Gravity acts all the time, and tends to bring about greater density 
wherever molecular movement permits. 

In addition to gravity, there are attractive forces between molecules, atoms, 


\ 


SECULAR MOVEMENTS 225 


ions, and electrons, which co-operate with gravity in accordance with laws of their 
own. Their general effect is to make matter denser. The extent of their opera- 
tion is undetermined, but there is ground for thinking that the density of the 
interior still may be increasing by their action. It is known that substances which 
crystallize in a given way under surface pressures may be changed into denser 
forms under higher pressures. Re-aggregation in the interior thus probably means 
increased density, and it may be going on constantly. While knowledge on this 
point is inconclusive, it is permissible to entertain the view that gravitational, 
molecular, atomic, and sub-atomic forces have been and are still at work tending 
to increase internal density. It is even conceived that this may be a chief (if not 
the chief) cause of earth-shrinkage. 

2. The resisting agencies. The condensing agencies are more or less held 
in check by resisting agencies. Of these heat is the most familiar. It is abetted 
by the molecular and atomic arrangements which exist at any given time, and 
which resist change, and by factors in the ultimate structure of matter, not well 
understood. It has been usual to regard the primitive state of the earth as one of 
intense heat, and to assign its subsequent reduction of volume almost solely to 
loss of heat; but this is not the view here favored. On the contrary, the heat of 
the earth is supposed to have been developed chiefly by reduction of volume and by 
radio-activity, and the heat thus developed is one of the forces which check further 
decrease of volume. Loss of heat is, of course, a cause of shrinkage, but its effect 
is thought to be less than that of molecular and sub-molecular rearrangements of 
the material of the earth, resulting in greater density. The loss indeed may not 
be greater than the new heat generated in the shrinkage. 

Observed temperatures in deep excavations. As the earth is penetrated below 
the zone of seasonal changes, by wells, mines, tunnels, and other excavations, the 
temperature is almost invariably found to rise, but the rate of rise is far from 
uniform. If we set aside as exceptional the unusually rapid rise near volcanoes 
and in other localities of obvious igneous influence, the highest rates are more than 
six times the lowest, the range being from 1° F. in 20 feet, to 1° in 135 feet,! with 
an average of 1° in 50 to 60 feet. The recent deep borings in which temperatures 
have been carefully recorded, indicate a slower rate of rise, say 1° for 80 feet. It 
is not probable that the observed rates of increase continue to the center. One 
degree in 60 feet, continued to the earth’s center would give a temperature of 348,- 
ooo° Fahr., and 1° Fahr. in 100 feet would give 209,000° Fahr. It is probable 
that the rate of increase diminishes with depth, and that the temperatures cited 
above are far in excess of those actually existing at the center. 

Amount of loss of heat and shrinkage. The amount of loss of interior heat may 
be estimated from that which is observed to be passing outward through the rocks, 
or by computations based on the estimated temperature gradients and with the 
known conductivity of rock. Estimates of the loss of heat in 100,000,000 years 
range from 10°C. (18° Fahr.) (Tait) to 45° C. (81° Fahr.), for the whole earth. 
This is an exceedingly small result, and emphasizes the low conductivity of rock. 
With this amount of cooling, the shrinkage resulting has been calculated. 
For a loss of 10° C., the circumferential contraction is calculated to be 1.6 to 2.35 
miles; for a loss of 45° C., 7.27 to 10.5 miles. These results are so small (cf.p. 223) 


11° F. for 250’ down to 8,000 feet, is reported from the Rand.,S. Af. Mining 
World, Jan. 7, 1911, p. 2. 


226 MOVEMENTS AND DEFORMATIONS 


that unless there is serious error in the estimates, cooling would seem to be a very 
inadequate cause for the shrinkage implied by mountain folds, overthrust faults, and 
other crustal deformations. This inadequacy has been urged strongly by various 
students of the problem.!' In view of the apparent incompetency of external loss 
of heat, the possibilities of distortion from other causes deserve consideration. 

Shrinkage from denser rearrangement of material already has been referred to 
(p. 225), and the transfer of heat from deeper to more superficial parts will be 
discussed in Chapter X.. A lowering of the average temperature of the inner 
half of the earth 500° C., and a raising of the temperature of the outer half an 
equal amount, would cause a lateral thrust of about 83 miles. Some transfer of 
this kind is among the theoretical possibilities under the planetesimal hypothesis. 
The process could not continue indefinitely; but computations imply that it still 
may be in progress. 

The rise of lavas. Vf lavas are forced out from beneath the surface, a com- 
pensatory sinking of the outer shell will follow. The great lava-flow of the Deccan 
is credited with an area of 200,000 square miles, and a thickness of 4,000 to 6,000 
feet. This would form a layer about 5 feet thick if spread over the whole surface 
of the globe. The compensatory sinking would cause a lateral thrust, on any 
great circle, of about 31 feet only. It requires a very generous estimate of the 
lavas poured out since the beginning of well-known geological history to cause a 
horizontal thrust amounting to any appreciable part of that involved in the folding 
of a typical mountain system. The case is different, however, if we go back to 
Archean times when the amount of extrusion was very large. Notable distortion 
- may have arisen from the extravasation of the lavas of that era. 

Intrusions of lava rising from lower to higher levels in the earth would have a 
dynamic effect similar to that of extrusions, so far as the outer part of the earth 
is concerned, and the amount of intrusive rock is probably far greater than that 
of extrusive. 

There are other possible factors in deformation which will not be discussed 
here. 


References on crustal movements. 

Dana, Manual of Geol., 4th ed., p. 345 et seq.; Willis, The Mechanics of the 
Appalachian Structures, 13th Ann. Report, U. S. Geol. Surv., Pt. IT (1893), pp. 
211-282; LeConte, Theories of Mountain Origin, Jour. Geol., Vol. I (1893), p. 542; 
Gilbert, Jour. Geol., Vol. III (1895), p. 333, and Bull. Phil. Soc. of Washington, 
Vol. XIII (1895), p. 31; Van Hise; Estimates and Causes of Crustal Shortening 
Jour Geol., Vol. VI (1898), U. S. Geol. Surv. (1904), pp. 924-931; A. Geikie, Text- 
book of Geology, 4th ed., pp. 672-702; Chamberlin and Salisbury, Geologic Proc- 
esses and their results, Chapter IX; R. T. Chamberlin; The Appalachian folds 
of Central Pennsylvania, Jour. Geol. Vol. XVIII (1910) pp. 228-251. 

Map work. See Plates CLXV to CLXVII of Professional Paper 60, U. S. 
Geol. Surv., and Exercise XVII, Interpretation of Topographic Maps. 


1 Fisher, Physics of the Earth’s Crust, Chap. VIII; and Dutton, Penn. M onthly, 
Philadelphia, May, 1870. 


Pernt ete, 


CHAPTER IX 
VULCANISM 


Vulcanism is the term applied to all movements of lava toward 
the surface of the earth, and is made to include certain other phe- 
nomena closely connected with these movements. In its rise, some 
lava reaches the surface, giving rise to eruptive or volcanic phenom- 





sae 


Fig. 221. A dike two feet wide, cutting through sandstone. Arran, coast ot 
Scotland. (H. M. Geol. Surv.) 
ena; and some intrudes itself into the outer formations of the earth 
and congeals there. The first gives rise to volcanic rocks, and the 
second to plutonic. ‘The first are extrusive; the second, intrusive; the 
first constitute eruptions; the second, irruptions. The fundamental 
nature of the two phases of vulcanism is the same. 


229 


228 VULCANISM 


I. INTRUSIONS 


Fluid rock forced into fissures and solidified there forms dzkes 
(Fig. 221); forced into chimney-like passages it forms pipes or 
plugs; insinuated between beds of other sorts of rock, it forms 
sills; and accumulated in considerable bodies which arch the strata 
up over them, it forms /accoliths (Fig. 222). If it breaks and lifts 
its cover, instead of arching it up, it is a bysmalith. Some laccoliths 
and bysmaliths are large enough to make good-sized mountains, 
of mound-like form. The Henry Mountains of Utah are laccoliths. 
Still more massive intrusions of igneous rock are sometimes called 
batholiths. The very great bodies of granite in Canada and along 
the axes of some of our western mountains are examples. The 
total amount of lava which has risen toward but not to the surface 
probably far exceeds all that has flowed out at the surface. In- 
trusions are usually seen only after erosion has removed the rocks 
which overlay them. 

There appear to be cases where intrusions come so near the 
surface as to develop explosive phe- 
nomena at the surface. At any 
rate, it is certain that occasional 
violent explosions take place where 
no lava comes to the surface. The 
explosion may be due to an intrusion 
of lava, or it may be due to the pene- 








Fig. 222. Ideal cross-section of . 
a laccolith with accompanying tration of surface-waters to hot rocks 


sheet and dikes. (Gilbert, U.S. that have remained uncooled from 


Geol. Surv. A ; : 
) previous volcanic action. A case 


of this kind occurred in Japan in 1888, where there was a sudden 
and violent explosion which blew away a considerable part of the 
side of a volcanic mountain which had not been in eruption for at 
least a thousand years. The explosion filled the air with ashes and 
debris like a violent volcanic eruption. There was but one eruption, 
and within a few hours the cloud of dust had disappeared and the 
phenomenon was ended. No lava was extruded. 
Intruded igneous rock changes the rock into which it is forced. 
Thin dikes and sills produce little effect, but greater masses alter 
the adjacent rock notably. The metamorphism is effected by (1) 
the heat, (2) the pressure incident to the intrusion, and (3) the 
chemical changes stimulated by the heat, water, and gases issuing 
from the lava, and by pressure in the presence of ground-water. 


EXTRUSIONS 220 


2. EXTRUSIONS 


When molten rock is forced to the surface it gives rise to the 
most impressive of all geological phenomena. The energies acquired 
in the interior under great compression here find sudden relief. En- 
closed gases may expand with extreme violence, hurling portions 
of lava to great heights and shattering them into fragments, special 
forms of which are called bombs, cinders, ash, etc., all of which con- 
stitute pyroclastic material. Much of the explosive violence of 
volcanoes has been attributed to the contact of the hot rising lava 
with ground-water. 

There are two phases of extrusion, and at their extremes they 
are contrasted strongly. The one is explosive ejection, attended 
in some cases with great violence; the other a quiet out-welling of the 
lava. More or less closely related to these two phases of extrusion 
are two classes of conduits, the one, restricted openings, such as 
pipes, ducts, or limited fissures, from which the amount of lava ex- 
truded is relatively small and forms cones; the other, great fissures 
out of which the lava pours in great volume and from which it 
spreads widely. ‘The extent of the spreading of lava into thin sheets 
is due more to the mass and fluidity of the lava than to the form 
of the outlet. The stupendous outflows of certain geologic periods 
appear to have issued mainly from extended fissures. 


Fissure Eruptions 


The chief known fissure eruptions of recent times are the vast 
basaltic floods of Iceland; but at certain times in the past there 
have been prodigious outpourings of lava, flow following flow, 
making formations thousands of feet thick and covering thousands 
of square miles. One of these occurred in Tertiary times in Idaho, 
Oregon, and Washington (Fig. 223), where about 200,000 square 
miles were covered with lava, aggregating in places some 2,000 feet 
in thickness. Still earlier, in the Cretaceous period, there were 
enormous flows on the Deccan, covering a like area to the depth of 
4,000 to 6,000 feet. Still earlier, in the Keweenawan period, an 
even more remarkable succession of lava-flows in the Lake Superior 
region developed a series of igneous rocks of almost incredible thick- 
ness. In these cases there is little evidence of explosive or other 
violent action, and little pyroclastic material. For the most part 
these wide-spreading flows are composed of basic material. Massive 
outflows of this class are the greatest examples of extrusions, 


230 VULCANISM 


though not now the dominant type. It has been thought that the 


A volcano is a cCir- 
cumscribed vent in the 
earth’s crust, out of which 
hot rock, gases, and va- 
pors issue. The ejected 
material is generally built 
up into mounds or cones 
(Figs. 224-225), which are 
often called volcanoes, 
though they are really 
the products of volcanoes. 

Fig. 223. Lava-flows of the northwestern So long as a volcano is 
part of the United States. active there is likely to 
be a depression, or crater (Fig. 226), in the top of its cone. The 
crater connects downward with the source of lava at unknown 


ose cose 0 OO ra oes 








Fig. 224. Cinder cone forming the summit of Mt. Vesuvius. 


depths. Craters may be a mile or more across, but most of them are 
smaller, some much smaller. After sufficient erosion, extinct vol- 





DISTRIBUTION OF VOLCANOES 231 


canoes show that the former passageways leading down toward the 
sources of lava vary much in size and shape. 

The exact number of volcanoes now active is not known, because 
most volcanoes are active periodically only, and it is impossible to 
say whether a volcano which is now quiescent is extinct or only 
resting. It is safe to include 300 in the active list, and the number 
may reach 350 or more. The number that have been active so 
recently that their cones remain distinct is several times as great. 


Mavna Loa 





Scale of ‘Tiles 
eo So 


Fig. 225. Profile of the cone of Mauna Loa. Vertical scale same as horizontal. 
(U. S. Geol. Surv.) 





Fig. 226. Sketch of the crater of cinder cone near Lassen Peak, Cal., showing 
the peculiar feature of two rings. The funnel is 240 feet deep. (U.S. Geol. Surv.) 


Distribution. 1. J time. In the earliest known ages, igneous 
action appears to have been very widespread. No great area of 
the oldest (Archean) rocks is known where the formations are not 
largely igneous. From the Paleozoic to the present, the distribu- 
tion of volcanic action over the surface seems to have been, in.a 
general way, much what it is to-day; that is, certain areas were 
affected at times by volcanoes, while other and larger areas had few 
or none. This is not equally true of all periods, as will be seen in 
the historical studies that follow. There were periods when vol- 
canic activity was widespread and energetic, and others when it was 
limited. The known facts do not indicate a steady decline, but 
rather a periodicity; at least this is so for the portion of the globe 
that is now known well enough to warrant conclusions. 

2. Relative to land and sea. Active volcanoes are located 
chiefly along the borders of continents, and within great oceanic 
basins (Fig. 227). On this account, the sea-water was formerly 
supposed to have some causal connection with their activity, and 


232 VULCANISM 


the presence of chlorine in the volcanic gases has been urged in sup- 
port of this view. Volcanoes, however, are not distributed so 
equably and exclusively about the several oceans as to give this 
conclusion force. Volcanoes are numerous within and around the 
Pacific, the greatest of the oceans, and in and around the Mediter- 
ranean, a much smaller body of water; but they are not especially 
abundant in or about the Atlantic. On the other hand, there are 
existing or very recent volcanoes in the interior of Asia, Africa, and 
America. If volcanoes were dependent upon proximity to the sea, 
they should have had close relations to it in the past, as much as 
now; but in recent periods there has been much volcanic activity 
in western America, far from the sea, and in the heart of Asia and 
Africa. In older periods, it is still less clear that there was any con- 
nection between volcanoes and oceans. 

3. Relative to crustal deformations. The distribution of present 
and recent volcanoes is more suggestively associated with those 
portions of the crust that have undergone movement in comparatively 
recent times, or are still moving. ‘The great mountain belt stretching 
from Cape Horn to Alaska and thence onwards along the east coast 
of Asia is dotted with active and recently extinct volcanoes. The 
tortuous zone of mountainous wrinkles about the Mediterranean, 
and thence eastward to the Polynesian Islands, is another notable 
volcanic tract. These two belts include the greater number of 
existing and recent volcanoes on the land. 

4. In latitude. Volcanoes appear to have no specific relation 
to latitude. Mounts Erebus and Terror amid the ice-mantle of 
Antarctica, and Mount Hecla in Iceland, as well as the numerous 
volcanoes of the Aleutian chain, give no ground for supposing that 
volcanoes shun the frigid zones, while the numerous volcanoes of the 
equatorial zone imply that they do not avoid the torrid belt. 

5. In curved lines. In the Aleutian and Kurile Islands, and 
elsewhere, there is a linear arrangement of volcanoes, with appre- 
ciable curvatures, the convexities of which are turned toward the 
adjacent ocean. In other cases there is a linear arrangement with- 
out appreciable curvature, as in the Hawaiian range. In some cases, 
volcanoes are bunched irregularly, as in some of the groups of vol- 
canic islands of the Pacific (Fig. 227). 

The relations of volcanoes. A significant feature in connection 
with volcanoes is the apparent sympathy between adjacent vents 
‘In some cases, and their entire independence in others. The recent 


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234 VULCANISM 


(1902) outbursts in Martinique and St. Vincent, and the symptoms 
of activity at the same time in other places, seem to point clearly to 
sympathy. On the other hand, the independence of some neighbor- 
ing vents, as those of Mauna Loa and Kilauea in Hawaii, is extraor- 
dinary. These two volcanoes are only about twenty miles apart, 
the one on the top and the other on the side of the same mountain 
mass. The crater of Loa is about 10,000 feet higher than that of 
Kilauea, and yet, while the latter has been in constant activity as 
far back as its history is known, the former is periodic. The case 
is the more remarkable because of the greatness of the ejections. 
The outflow of Mauna Loa in 1885 formed a stream 3 to ro miles in 
width and 45 miles in length, with a probable average thickness of 
too feet, and some of its other outflows were nearly as massive. 
Besides this massiveness, there were extraordinary movements of 
the lava within the crater, if the testimony of witnesses may be 
trusted. But throughout these great movements in the higher 
crater, the lava-column of Kilauea, 10,000 feet lower, continued its 
quiet action without sensible relation to its boisterous neighbor. 
No difference in specific gravity that could account for a difference 
in height of 10,000 feet has been observed or can be presumed. It 
seems a necessary inference, therefore, that the lava-columns in 
the two volcanoes have no connection with each other, or with a 
common reservoir. The tops of some lava-columns stand about 
20,000 feet above the sea, while others emerge on the sea-bottom 
far below sea-level. ‘This range of elevation tells its own story as 
to the independence of vents. 


Eruptions seem to be somewhat more common when atmos- 


pheric pressure is high than when low, doubtless because the in- 
creased atmospheric weight on a large area of the crust, aids in forc- 
ing out lava and volcanic gases. This can be effective only when 
other forces have almost accomplished the result. Eruptions seem 
also to be more common when tidal strains favor them, for like rea- 
sons. In the same class are probably to be put the effects of heavy 
rains. Such factors are regarded as mere incidents, of no moment 
as real causes of vulcanism, but of some value in determining the 
moment of eruption. 


Periodicity. Most volcanoes are intermittent in their action, © 


long periods of dormancy intervening between periods of activity. 
Some volcanoes supposed to be extinct have renewed their activity 
with terrific violence. Their periodicity awaits an explanation, 


eRe 2 oe 


PRODUCTS OF VOLCANOES 235 





Fig. 228. The edge of an old stream of lava, showing (1) its broken character 
due to movement after the outside had hardened, and (2) the steep slope of the 
stream of stiffened lava. Near Flagstaff, Ariz. (Fairbanks.) 
but the temporary quiet very likely means an exhaustion of the 
supply of gas or lava, or both. 

Products of volcanoes. 1. Pyroclastic material. The fragmental 
materials which are blown out of a volcano are, as a rule, portions 
of lava which solidified before ejection, or during their flight in the 
air. From masses of rock tons in weight, the fragments grade down 
to particles of dust. The dust particles (often called ask) are thrown 
high into the air in some cases, and, caught by the winds, are shifted 
incredible distances (p. 13). In some cases, beds of volcanic ash 
many feet in thickness (as those of Nebraska) are found far from any 
known volcanic center. The extremely fine ash from the great 
explosion of Krakatoa floated several times around the earth in the 
equatorial belt, and spread northward into the temperate zones. 

Liquid rock, lava. The term lava is applied to all kinds of liquid 
rock, and also to the solid rock formed when fluid rock congeals. 
The various phases assumed by lava, on solidification, have been 
noted in connection with igneous rocks. Lava never flows so freely 
as water, and is, in many cases, very stiff or viscous. The distance 
to which it flows depends on its liquidity, its amount, and the slope 
ef the surface on which it is poured out. 


‘ 


236 VULCANISM 


As lava flows, its upper surface may cool so much as to become 
hard while the interior is still fluid. The fluid part may then 
break out at the side or end of the hardened shell and flow away, 
leaving a hollow crust of solidified lava. On further cooling, the 
shell contracts and cracks, and perhaps caves in. The hardened 
surface of a lava-flow may be broken by the movement of the fluid 
lava below, and the solid fragments be displaced and upturned 
so as to give the surface a jagged appearance. 

3. Gases and vapors. The gases and vapors which issue from 
volcanoes are of many kinds. Among the commoner ones are those 
of water (H2.O), carbon dioxide (COs), carbon monoxide (CO), 
chlorine (Cl), hydrochloric acid (HCl), sulphur dioxide (SOz), and 
hydrogen sulphide (H2S); but with these more important ones there 
are many others. Oxygen and hydrogen are generally present, per- 
haps produced by the dissociation of the elements of water. Some 
of the gases are poisonous, and, as in the case of Pelée, their tem- 
perature is in some cases so high as to be destructive to life. 

Formation of lava cones. Lava usually flows away from a vent 
in streams which solidify before running far. As the lava-streams 
flow in different directions at different times, the total effect is a low 
cone formed of radiating tongues of lava. The streams may congeal 
before they reach beyond the base of the cone, and not rarely while 
yet on its slope. The volcanic cones formed of lava have low slopes, 
since the fluidity of the lava prevents the development of high 
gradients. It is, however, the exception rather than the rule, that 
the cone is made up mainly of lava-streams, though the great Hawai- 
ian volcanoes are of this class. The form of the cone, when com- 
posed chiefly of lava, is also affected by the mass of the outflow and 
by the fluidity of the lava. Other things being equal, the larger 
the outflow at a given time, the more widely it distributes itself, 
and the flatter the cone. 

Cinder-cones. ‘The larger portion of the lava blown into the air 
by expanding gas-bubbles falls back in the immediate vicinity of 
the vent and builds up cinder-cones. This fragmental matter may 
be disposed more or less symmetrically, making a cone with 
steep slopes (Fig. 224). 

Minor cones. Small or temporary vents formed as offshoots 
from the main vents may give rise to secondary or ‘‘parasitic’’ 
cones. These may be numerous, as in the case of Etna, and so 
important that a volcanic mountain becomes a compound cone. 





VOLCANIC CONES 237 





Fig. 229. Spatter-cone and cavern. Kilauea, Hawaii. (Photo. by Libbey.) 


A still more subordinate type of cone is the ‘‘spatter-cone”’ formed 
about small vents that eject little dabs of lava which form chimneys, 
cones, domes, etc. Spatter-cones (Fig. 229) may arise from the 
surface of the lava-flows themselves. 

From most existing volcanoes both lava-flows and fragmental 
ejecta are given forth, and the resulting cones are composite in 
material. Lava breaks through the side of a cone more frequently 
than it overflows its summit, and this gives rise to irregularities of 
form and structure. Cones also are subject to partial destruction 
both by outbursts of lava and by explosions. As a result, many 
volcanic regions show old, partially destroyed craters, as well as new 
and more perfect ones. 

In violent eruptions, steam, accompanied with much ash, is 
shot up to great heights, rolling outwards in cumulus or cauliflower- 
like forms (Fig. 230). In the more violent explosions, these columns 
are projected several miles. In the phenomenal case of Krakatoa, 
the projection was estimated at seventeen miles. By reason of 
its great expansion as it rises, and by its contact with the colder 
air, steam is condensed quickly, and prodigious floods of rain 
accompany many an eruption. This rain, carrying down a portion 
of the ash and gathering up much that had previously fallen, gives 


238 VULCANISM 


rise to mud-flows, which in some cases constitute a large part of 
the final deposit. These mud-flows lodge chiefly on the lower 
slopes of the cone or adjacent to its base. 


The common view that lava is melted rock, is hardly the correct one. At 
any rate, it is at least equally correct to regard it as a solution of mineral matter 
in mineral matter. A familiar illustration will show what is meant. If ice and 
salt are mixed at a temperature of 30° F., the two form a liquid, though the tem- 
perature is too low to melt either. We say the salt is dissolved, but it would be 





Fig. 230. The eruption cloud of Pelée, December 16, 1902. (Lacroix.) 





NATURE OF LAVA 239 


just as correct to say that the ice is dissolved. The two minerals, ‘ce and salt, 
are dissolved in each other, and the solution takes place at a temperature below 





Fig: 231. Relatively smooth lava surface near the Jordan craters, Malheur Co., 
Ore. (U.S. Geol. Surv.) 





Fig. 232. Ropy surface of lava, Mauna Loa, flow of 1881. (Calvin.) 


240 VULCANISM 


the melting point of either. Something of the same sort appears to take place 
when rock becomes liquid. The distinction between such solutions and molten 
rock is not very sharp, but it is essential to know that the order in which the min- 
erals crystallize from lavas is not. dependent on their melting temperatures. It 
appears rather to depend on the order in which the solution becomes saturated 
with the constituents of each of the several minerals. For example, quartz, which 
has a very high melting-point, may crystallize out from a lava much later than 
minerals which have lower melting temperatures. The solutions are exceedingly 
complex, and include a wide range of chemical substances. Chief among them 
are silicates of aluminum, potassium, sodium, calcium, magnesium, and iron 
(Chapter X), with minor ingredients of nearly all knownsubstances. Gases as well 
as rock materials enter into the composition of the igneous rock. When lava is 
cooled suddenly, the result is glass, every part of which has essentially the same 
composition that the liquid had, but even in this case some of the gases of the 
lava escape. If the cooling is slower, the various substances in the mixture crys- 
tallize out into minerals in the order in which they severally reach saturation. 
This involves the principle that solubility is dependent on temperature, and that 
as the temperature sinks the degree of solubility declines, and the saturation-point 
for some constituents of the solution is reached earlier than that for others. With 
sufficiently slow cooling, all the material passes into the solid state by the crys- 
tallizing of the several minerals in succession. This does not mean that two or 
more minerals may not be forming at the same time, but it means that some 
minerals may be crystallized out while the surrounding material is still fluid. In 
most igneous rocks, nearly perfect crystals of certain minerals are common, while 
other minerals, crystallizing later, adapt themselves to the space left between older 
crystals. This conception is supported by the fact that some lavas, while still in the 
fluid condition, contain well-formed crystals, very much as water in certain 
conditions may be filled with crystals of ice. 

Temperature of lava. Accurate determinations of the temperatures of liq- 
uid lavas have not been made; but it is clear from the white heat of some lavas 
that their temperatures are appreciably above the melting-point. This is 
also a necessary inference from the length of time lavas remain fluid, in spite of its 
contact with cooler rock, through its miles of ascent. From various facts it is 
probably safe to assume that the original temperatures of lavas as they rise to 
the surface are in some cases considerably above 2,000° Fahr. (1,093° C.). Even 
such a temperature must be somewhat below the original temperature of the lava, 
because some heat must be lost in rising, both by contact with the cooler rocks 
through which it rises, and by the expansion of the gases within them. 

Depth of source. Attempts have been made to determine the depth from 
which lavas rise, by calculations based on the earthquake tremors accompanying 
eruptions; but such calculations really tell very little concerning the true point 
of origin of the lava. At most they probably tell merely where the ascending 
lavas begin to rupture the rock through which they pass, and rupture may not be 
possible below the zone of fracture,which is probably not more than eleven miles deep.! 
In the zone of flowage below, where the pressure is too great to permit fracture, the 
lava not improbably makes its way by some boring or fluxing process, which 
might not be capable of giving rise to seismic tremors. ‘The tremors perhaps com- 


1Adams: Jour. Geol., Vol. xx (1912), pp. 97-118. 


—~ 


 —a 


NATURE OF LAVA 241 


pel us to place the beginning of movement of lava at least as low as the bottom of the 
fracture zone, but they probably offer no sufficient ground for limiting the lava’s 
origin to this or any other specific depth. 

Volcanic gases. One of the most distinctive features of volcanoes is the 
explosive action arising from the gases and vapors pent up in the lava. Lavas 
in the interior, under high pressure, contain much gas, and as they rise and the 
pressure is relieved, some of these gases escape from the hot liquid. In those 
cases in which the eruption is quiet, the escape of the gases is but partial while 
the lava is in the crater, and much gas remains to be given off after the lava 
has been extruded and is about to congeal. The gases are then given off slowly 
and quietly. If, however, the lava is surcharged with gases, and if their escape 
is retarded by the viscosity of the lava, they gather in large vesicles or bubbles 
in the lava in the throat of the volcano, and on coming to the surface explode, 
hurling the enveloping lava upwards and outwards. The violence of the explo- 
sion reduces a portion of the lava to the fineness of dust,— the “ash” and 
“smoke” of the volcano. 

The causes of the differences of gas action in different volcanoes are undeter- 
mined, but the following suggestions may point to a part of the truth: (1) Some 
lavas contain more gases than others, and hence are predisposed to be more ex- 
plosive; (2) some are more viscous than others and hence hold the gases more 
tenaciously until they accumulate and acquire explosive force, while the more 
liquid lavas allow their gases to escape more freely; (3) probably a main occasion of 
violent explosions lies in the fact that the lavas have begun to crystallize while 
yet in the volcano. When crystals form in the magma (lava), they exclude the 
gases which were in the substance from which they are developed, and this excluded 
gas overcharges the remainder of the lava. This view is supported by the fact 
that the pumice and ash of such extraordinary eruptions as those of Krakatoa 
and Pelée contain many small crystals which had formed before the explosion took 
place. Incipient crystallization does not, however, appear to be a_ universal 
accompaniment of explosive action. 

Igneous rocks contain gases in large quantities. When the lavas lodge under- 
ground without free communication with the surface, there is reason to think that 
they retain a larger percentage of their original gases than the lavas which are 
exposed freely at the surface. At any rate, deep intrusive rocks contain notable 
quantities of gases. Recent surface lavas also contain gases of similar kinds, but 
not in equal amount, so far as available analyses show. 

One of the outstanding problems of geology is to determine (a) how far the 
material of the gases had the same origin as the material of the lavas, and (b) how 
far the material for the gases penetrated from the surface. The peculiar propor- 
tions of the rock-gases, among which hydrogen and carbon dioxide greatly pre- 
ponderate, seem to imply that they are not derived chiefly from surface waters or 
the atmosphere; they appear to be original constituents of the rocks in the main, 
and when given forth they appear to constitute real additions to the atmosphere. 


THE CAUSE OF VULCANISM 


The fundamental explanation of volcanic phenomena is wrapped 
up in the origin of the earth, for the conditions which the earth in- 
1 Rollin T. Chamberlin, Gases in Rocks, Carnegie Institution, 1908. 


242 VULCANISM 
herited from its birth are doubtless leading factors in the explana- 
tion of vulcanism.! The explanation includes (1) the origin of 
lavas, and (2) the forces by which they are expelled. 

The current explanations of vulcanism fall into two general 
classes: (1) those which assume that lavas are residual portions of 
an original molten mass, and (2) those which assign lavas to the local 
liquefaction of rock. The first of these views prevailed formerly, 
but it encounters grave difficulties because of the independent action 
of adjacent vents. When lava columns vary thousands of feet in 
height on the same mountain mass, as in the Hawaii volcanoes, 
even a resort to the hypothesis of local residual reservoirs is un- 
satisfactory. 

Another view which has had much currency supposes that 
surface water and its absorbed gases penetrate to heated rock and are 
absorbed by it, rendering the whole liquid, and that the lava thus 
formed is forced to the surface. It does not appear, however, that 
surface water penetrates below the zone of fracture, and hence is far 
from reaching highly-heated rocks. Relief of pressure lowers the 
melting point of rock, and when felt by rocks already hotter than 
their melting temperatures at lowered pressures, has been held to 
be a possible cause of vulcanism. The necessary relief of pressure 
is assigned to faulting and denudation; but many volcanoes are 
located in the bottom of the ocean, where denudation does not take 
place, and faulting that would give relief of pressure is not always 
related to vulcanism in any clear way. Melting by crushing has been 
suggested, but in the deeper parts, crushing involves increase of 
pressure, which opposes melting. Sinking to the zone of high tem- 
perature under the weight of accumulated sediments, is also assigned 
as a cause of melting, but there is very little sedimentation in the 
ocean far from land where many volcanoes are situated. 

If the earth grew up by slow accessions of matter, and if its 
interior heat is due chiefly to the internal compression resulting 
from growth, the distribution of internal temperature would be such 
that, with like conductivity, the flow of heat from the deep interior 
to a thick outer zone (about 1/5 of the radius) of the earth would 
be greater than the loss from this zone to the superficial shell. 
The deeper parts of the outer zone might thus rise in temperature. 
This zone is, under this view, supposed to be composed of various 


For fuller statement see the authors’ larger work, Vol. I, pp. 395-607, Vol. IT, 
Pp. 99-106, 116-118, 120, 130. 


— 


: 
| 
: 





CAUSES OF VULCANISM 243 
kinds of matter, mixed as they happened to fallin. If its tempera- 
ture rises, the fusion-points of some of its constituents will be reached 
sooner than those of others. A fusion or solution of the more soluble 
portions may thus take place while the rest of the rock remains 
solid. The gases and volatile constituents in the original material 
would obviously unite with the liquid part. With continued rise 
of temperature, the liquefaction would extend itself until adjacent 
pockets or threads of lava 
united, and the lighter portions SIS SwS 
of the fluid would be forced 5 ween Ginny yt oko Tp 
upward and would work their \4\\% 
way toward the surface by fus- 
ing and fluxing. 

As the lavas rise, the pres- 
sure on them becomes less, and 
hence the temperature neces- 
sary for liquefaction gradually 
falls, leaving them a constantly 
renewed margin of temperature 
available for melting their way 
through the upper horizons. 
Thus it is conceived that these 
fusible and fluxing selections 
from the middle zone might 
thread their way up to the zone 
of fracture, and thence, taking 
advantage of fissures and 





Fig. 233. Ideal section of a portion of 
the early earth, illustrating its assigned 
modes of vulcanism. C, center; S, sur- 
face; a-a’, fragmental zone; a’-f, zone of 


cracks, reach the surface (Fig. 
233). It is conceived that such 
liquefaction and extrusion 
would carry the excess of tem- 
perature received by the lower 
part of the outer zone toward 
the surface, or even out to it. 
The outward movement of the 


continuous rock below melting tempera- 
ture at the surface; ff-c, interior portion 
whose temperatures rise from the surface 
melting-point at f-f to a maximum at C; 
V, V, threads or tongues of molten rock 
rising from the interior to various levels, 
many of these lodging within the frag- 
mental zone as tongues, batholiths, etc.; 
PPP, explosion pits formed by volcanic 
gases derived from tongues of lava below. 


lava would tend to lower the temperature below, forestalling gen- 
eral liquefaction, and keeping the zone as a whole, solid. The 
independence of volcanoes is assigned to the independence of the 
liquid threads that work their way to the surface. Nothing like a 
reservoir or molten lake enters into the conception. The prolonged 


244 VULCANISM 


action of volcanoes is attributed to the slow feeding of the liquid 
threads from the middle zone, which is liquefied in spots only. The 
frequent pauses in volcanic action are assigned to temporary defi- 
ciencies of supply, and the renewals to the gathering of new supplies 
after a sufficient lapse of time. The distribution of volcanoes in 
essentially all latitudes and longitudes is assigned to the general 
nature of the cause. The special surface distribution is assumed to 
be influenced, though not altogether controlled, by the favorable or 
unfavorable conditions for escape of lava to the surface. The per- 
sistence of volcanic action in time is attributed to the magnitude 
of the interior source, to its deep-seated position, and to the slowness 
of conduction of heat from the earth’s interior. The force of expul- 
sion is found in the stress-differences in the interior, particularly 
the periodic tidal stresses, and in the slow pressure brought to bear 
on the slender threads of liquid by the creep of the adjacent rock. 
The violent explosions are due to the included gases, of which steam 
is chief. Little efficiency is assigned to surface-waters, and that little 
is regarded as secondary and incidental. The true volcanic gases 
are regarded as coming from the deep interior, and as being, after 
expulsion, accessions to the atmosphere and hydrosphere. The 
standing of the lavas in volcanic ducts for hundreds and even 
thousands of years with only little outflow, as in some of the best- 
known volcanoes, is regarded as an exhibition of an approximate 
equilibrium between the hydrostatic pressure of the deep-pene- 
trating column of lava and the flowage-tendency of the rock-walls, 
the outflow being also conditioned on the slow supply below, and 
on the periodic stress-differences of the interior. 

For the present, volcanic hypotheses must be left to work out 
their own destiny, serving in the meantime as stimulants of research. 
All but the last have been long under consideration. The recent 
discovery of the heating effects of radio-activity has given rise to 
the hypothesis that the origin of lavas is due to this cause. It 
seems clear that this must at least be a cooperative agency. It is 
too early in the new investigation to decide whether it can wisely 
be regarded as the sole cause or even an essential one. 

How lava reaches the surface. All views that locate the origin 
of the lavas deep in the earth must face the difficulty of the passage 
of lava through rock below the fracture zone. Near the surface, 
the lavas usually take advantage of bedding-planes, or of fissures 
already existing, or made by themselves. There is little evidence 


a 


CAUSES OF VULCANISM 248 


that they bore their way through the zone of fracture. In the 
denser and warmer zone below, the alternatives seem to be (1) 
mechanical penetration without fracture, or (2) melting or fluxing. 
As rocks “‘flow”’ in this zone by differential pressure without rup- 
ture, an included liquid mass may perhaps be forced to flow through 
the zone by differential pressure. Lava probably fuses or fluxes its 
way, under pressure, through the rock below the zone of fracture. 
In this it may be supposed to be assisted by its gases, by the selective 
nature of its fluxing, by its exceedingly high temperature if it comes 
from very great depths, and by the stress-differences which attend 
tidal strains in the deep interior. In ascending, the lava would be 
invading regions of lesser pressure and lower melting-point. It 
would therefore have heat in excess of the local melting temperature, 
until it reached the cool rock. From that point on, the rising lava 
must constantly lose temperature by contact with cool rocks. If. 
its excess of temperature is insufficient to enable it to reach the zone 
cf fracture, the ascending column is arrested and becomes plutonic 
rock. If it suffices to reach the zone of fracture, advantage may be 
taken thereafter of fissures, and the problem of further ascent 
probably becomes chiefly one of hydrostatic pressure, in which the 
ascent of the lava-column is favored by its high temperature and its 
included gases. The hydrostatic contest is here between the lava- 
column measured to its extreme base, and the adjacent rock-columns 
measured to the same extreme depth. The result is, therefore, not 
necessarily dependent on the flowage of the outer rocks, but may be 
essentially or wholly dependent on the deep-seated flowage of the 
rock of the lower horizons. The ascending column may reach 
hydrostatic equilibrium before it reaches the surface, and then 
form intrusions of various sorts, or it may find equilibrium only by 
coming to the surface. 


References. C. E. Dutton, Hawaiian Volcanoes, Fourth Ann. Rept., U. S. 
Geol. Surv., 1883. Judd, Volcanoes, 1881; J. D. Dana, Characteristics of Vol- 
canoes, 1890. A. Geikie, Ancient Volcanoes of Great Britain, 1897. I. C. Russell, 
Volcanoes of North America, 1897. T. G. Bonney, Volcanoes, Their Structure 
and Significance, 1899. A. Heilprin, Mont Pelée and the Tragedy of Martinique, 
1903. Accounts of same volcanoes in the Nat’l. Geog. Mag., Vol. XIII, 1902 
(Russell, Hill, Hovey, Diller, and Hildebrand). 

Map work. See Plates CLV to CLXIV, of Professional Paper 60, U. S. Geol. 
Surv. and Exercise XVI, in Interpretation of Topographic Maps. 


CHAPTER X 
MATERIALS OF THE EARTH AND THEIR ARRANGEMENT 


The general constitution of the lithosphere has been referred 
to already (p. 7), but we are now to study in more detail the 
nature, the arrangement, and the history of the rocks. The igneous 
rocks will be considered first. 


IGNEOUS ROCKS 


Appearanceatthesurface. Thepreceding chapter has acquainted 
‘us with the fact that some igneous rocks were extruded either from 
volcanoes or from fissures, and that extrusive rocks include both 
lava flows and pyroclastic materials. Under proper conditions, 
extruded rocks may be buried later beneath sediments, or may be 
worn away by erosion. It follows that only a part of the igneous 
rocks extruded in the past, and especially those of relatively recent 
times, remain at the surface. 

By removing the overlying rocks, erosion exposes the intruded 
rocks of dikes, sills, laccoliths, batholiths (p. 228), etc., and a 
considerable part of all accessible igneous rock is now at the surface 
because the rocks which overlay it have been worn away. The 
great areas of granite in Canada, and the long axes of many of our 
western mountains, are examples. Extruded igneous rock which 
has been buried, also is subject to subsequent exposure by the 
wasting away of its cover. 

Structural features of igneous rocks. The names applied to the 
principal forms of igneous intrusions imply certain large structural 
features; but igneous rocks have certain other structural features 
which distinguish them from other rocks. ‘Thus the rock of lac- 
coliths, bysmaliths, and batholiths is generally massive. This 
term means not simply that the rock occurs in large bodies, but 
that the rock has no distinct cleavage. It is not in beds, and it is 
not schistose. Sills and some extrusions of lava take on the form 
of sheets. Where one extrusive sheet of lava overlies another, the 
succession of sheets has some resemblance to stratified rock; but 

246 


4 <——_ei ne 


IGNEOUS ROCKS 247 


the rock of the individual sheets shows little indication of arrange- 
ment in layers. Some extruded rock has a structure developed by 
the flow of the lava after it had become stiff from cooling. This 





Fig. 234. Flow structure in volcanic glass. About half natural size. (Photo. 
by Church.) 





Fig. 235. Columnar structure in igneous rock. Giant’s Causeway. 


248 MATERIALS AND THEIR ARRANGEMENT 


is known as. flow structure (Fig. 234). On cooling, some lavas 
develop columnar structure (Fig. 235), the columns being roughly 
perpendicular to the surface of cooling. 
The explanation of the columns is probably somewhat as fol- 
lows: The surface of the lava contracts about equally in all direc- 
tions on cooling. ‘The contraction may be thought of as centering 
about equidistant points. About a given point, the least number 
of cracks which will relieve the tension in all directions is three 
(a, Fig. 236, A). If these radiate symmetrically from a point, the 
angle between any two is 120°, the angle of the hexagonal prism. 
Similar radiating cracks from other centers (0, c, etc.) complete the 
columns (Fig. 236, B). A five-sided column would arise from the 
failure of cracks to develop about one of the points. 
Igneous rocks are affected by cracks or joints, which run through 
them in various directions, 
but this is not a feature 
peculiar to igneous rocks. 
Pyroclastic rocks have 
somewhat the structure of 
ale sedimentary rocks. If the 

fragmental volcanic matter 
accumulates on the surface 
A B of the land, it may lack 


Fig. 236. Diagrams to illustrate the for- distinct stratification: but if 
mation of columns of basalt: A, the first : 


stage in the development of a hexagonal it falls or is washed into 
column. 8B, the completion of a hexagonal water, it may be assorted 


ee pai and stratified. In this case 
it is distinguished from other clastic rock by its constitution. 
Texturalfeatures. Most igneous rocks are made up of interlock- 
ing crystals of different sorts. These crystals may be so small that 
they are not distinguished readily by the eye, or they may be so large 
as to be seen easily, or some may be large and some small. If theyare 
large enough to be distinct to the eye even without close scrutiny, the 
rock is coarsely crystalline. All such rocks may be called phanerites. 
In. phanerites, the interlocking of the crystals is evident (Fig. 237). 
lf the crystals are so small as not to be seen readily by the eye, the 
rock is aphanite. In all igneous rocks, the crystals are of somewhat 
unequal size; but in some, there are certain crystals, usually of some 
one mineral, which are so much larger than the others as to be © 
conspicuous. The rock is then porphyritic (Fig. 238). The smaller 


IGNEOUS ROCKS 249 





crystals of one or two kinds of mineral; the dark parts represent crystals of others. 


crystals in which the larger ones are set may be so small as not to 
be readily distinguished (aphanitic), or they may be visible sepa- 


rately (phaneritic). 

Some igneous 
rock is, in reality, 
volcanic glass. Vol- 
canic glass (obsid- 
ian) is one phase of 
solidified lava. It 
is formed when the 
liquid lava solidifies 
quickly, before the 
crystals have time 
to grow. Some ig- 
neous rock is made 
up partly of glass 
and partly of crys- 





Fig. 238. Porphyritic texture. pp=phenocrysts of 
feldspar. The smaller crystals are of feldspar, mica, 
and quartz. (Watts.) 


tals, and between the rock which is all glass and that which is all 
crystals there are all gradations. Whether lava becomes glassy or 
crystalline on hardening, or whether it is partly the one and partly 


250 MATERIALS AND THEIR ARRANGEMENT 


the other, depends on the conditions under which it solidifies. All 
liquid lava contains the materials out of which crystals may be 
formed, under proper conditions. | 

Glassy and partly glassy rock may be compact or porous. 
Porous rock of the type shown in Fig. 239 is called scorzaceous. 





Fig. 239. Scoriaceous texture. About 4/5 natural size. (Photo. by Church.) 


Rock of this sort is really lava froth, solidified. The pores are the 
spaces occupied by gases when the lava hardened. Some of the 
bubbles were large and some small. Pumice is porous volcanic 
glass, the pores being small. 

Besides these varieties of texture which originate as lava hardens, 
there are the textures peculiar to pyroclastic rocks. When quanti- 
ties of volcanic dust, etc. (sometimes called volcanic ash), become 
coherent, as by cementation, the resulting rock is called tuff (or 
volcanic tufa). If the constituents are largely coarse, the resulting 
rock is volcanic agglomerate. 

Liquid lava (=Jiquid glass). Liquid lava is essentially fluid 
glass. Itis analogous to common glass, which is a silicate of potash, 
soda, or other base, except that manufactured glass is relatively 
free from iron and other coloring substances which abound in lavas, 
rendering them dark and more or less opaque. Lavas, too, are 


Fr | 





IGNEOUS ROCKS 251 


usually mixtures of several silicates, while manufactured glasses 
consist of only one, or at most a few. Furnace slag is essentially an - 
artificial lava. 

Solidification and crystallization. When lava is cooled quickly, 
its components solidify essentially as they were in the liquid; for 
there is no time for the molecules of one kind to come together in 
regular systematic order, as is necessary to'form crystals. In a 
thick viscid liquid, the arrangement of molecules into definite 
crystal forms takes place slowly. Because of this slowness, the 
solidification of the lava may catch the process of crystallization at 
any stage. This is why some igneous rocks are glassy (cooled 
quickly), some partly glassy (cooled less quickly), and some wholly 
crystalline. In general, the slower their growth, the larger the crystals. 

Stages of crystallization. Eruptions take place intermittently, 
and the lava beneath the surface may be cooling during the inter- 
vals between eruptions. After a certain stage of partial crystalliza- 
tion has been reached during such time of quiet, a new eruption may 
shift the whole mass of lava into new surroundings, anda second phase 
of solidification may be added to the first. The rock may then show 
two phases of crystallization: (1) large crystals of the kind or kinds 
developed during the first stage of slow subterranean cooling; and 
(2) small crystals or glass developed during the more rapid cooling 
of the second stage. The result is large crystals set in a matrix of 
small crystals or of glass. This is perhaps one way in which por- 
phyry is formed. 


Composition of Igneous Rocks 


Nearly all the chemical elements are found in igneous rocks, 
though but few of them are abundant. These few are regarded as 
the essential constituents, while the rarer substances are regarded as 
incidental. The relative amounts of the more abundant elements 
in the crust of the earth, as nearly as now known, are shown in the 
following table: 


Per cent in the 


Element Symbol Solid Crust 
Oxygen U5 AEA AE ee ae Ue ake AE: 0a Sa 47.02 
Silicon (oh) oie nny NPE RGIS ewe k 1S eo Aloe et in nr ee 28.06 
PRISCA)... io PE A ae. ERCP Ct 8.16 
Tron Be) eh et PR Ny Ait: SAR I SE NS 5. AG it ag 04 
Calcium “fe UR At ON ae its Dok aise |? a RNa he Mees 3.50 

- Sodium SC oe GE id Sag Bei Wt UAE | ant ase) ae £r4:63 


252 MATERIALS AND THEIR ARRANGEMENT 


Potassium) (K)..cei9 4. Sei e.45 Re Ws a ot 2.32 
Titanium «., (Ti) Aid. ga ee si 9 os Siew sh pan 41 
Hydrogen © (H) ooo. ie. ye tte kd © oale eee 17 
Carbon (Ol Beret te Cae RN .12 
Phosphorus (P).. fid5. ete tee. urbe > et 09 
Manganese’ (Mn) #2.) Su. n.000. UL Se .O7 
Sulphur (S) + sid debe al ive es eae eee "ae. ae .07 


It will be seen that only eight of the elements enter into the 
earth’s crust to the extent of one per cent, and no other one reaches 
half of one percent. Many elements that are of great importance 
in the affairs of men occur in quantities too small to be estimated in 
percentages. The precious metals, such as platinum, gold, and 
silver, and even some of the more common ones, as lead, zinc, and 
copper, are of little importance quantitatively. 


Union of elements. For present purposes we may neglect all but the first 
eight of the elements mentioned above. Out of these elements come various 
chemical combinations when the lava solidifies; out of these combinations come the 
various minerals; and from combinations of minerals come various kinds of rocks. 
The union of oxygen with the other seven elements may be taken as a funda- 
mental step in this series of combinations. The result is the following oxides: 
Silica (SiO2), alumina (Al,O3), the ferrous, ferric, and magnetic oxides (FeO, 
Fe.O03, and Fe.O3), magnesia (MgO), calcium oxide (lime) (CaO), soda (Na20), 
and potash (K2O). The oxygen sometimes unites in proportions different from 
those here given, but exceptions may be neglected here. 

Of these nine oxides, silica acts as an acid, or more strictly as an acid anhydride. 
All the rest except the magnetic oxide of iron, and sometimes the oxide of alumi- 
num, act as basic oxides. The proportion of silica in igneous rocks is so significant 
that all such rocks are sometimes grouped into three classes, as follows: those 
with more than 65% of silica are acidic; those containing 55 to 65%, intermediate; 
and those containing less than 55%, basic. 

The union of silica (SiO2) and lime (CaO) forms calcium silicate, CaO,SiOs, 
or CaSiO3. The union of silica and magnesia forms magnesium silicate, MgO,SiO», 
or MgSiO;. Corresponding unions of silica and the other oxides named, give rise 
to other silicates. 

Formation of minerals. Since but one of the leading oxides (silica) that 
abound in the average lava plays the part of an acid, a very simple conception of the 
general nature of igneous rocks may be reached by noting that they are com- 
posed mostly of silicates of the eight leading basic oxide—those of alumina, potash, 
soda, lime, magnesia, and iron. This general idea represents a most important 
truth; but in its use we must not forget that there are many exceptions. Sulphur, 
phosphorus, chlorine, and other elements unite with the bases to form sulphates, 
sulphides, phosphates, phosphides, chlorides, etc. So also there are many minor 
bases that form silicates; and these minor bases unite with minor acids to form 
many of the rarer minerals. Again, there are native metals in some igneous rocks; 
but altogether these minor compounds hardly reach more than one or two per 
cent of the whole, 


his aa...) - 


MINERALS OF IGNEOUS ROCKS 253 


There are two exceptions of more importance. In the liquid lava the acid and 
basic elements are not always evenly matched. When there is an excess of silica, 
a portion remains free and takes the form of quartz (SiO2). If there is an excess 
of the basic oxides, the weakest one is usually left out of the combination. This is 
commonly an iron oxide (FesO4), called magnetite. It is a singular fact that quartz 
forms in some cases where there is no excess of silica, and magnetite where there 
is no excess of base. Quartz (free acid anhydride) and magnetite (free basic oxide) 
may occur in the same rock. The explanation of this is yet to be found. The 
oxides of silicon and iron form rather important exceptions to the general state- 
ment that igneous rocks are made up mostly of silicates; but, thus qualified, the 
statement expresses the essential truth. 

But here simplicity ends, and the sources of complexity are several. In the 
first place, silica unites with the bases in different ratios, and thus gives rise to 
uni-silicates or ortho-silicates (ratio of oxygen of base to oxygen of silica, 1:1), 
sub-silicates (the above ratio more than 1), bisilicates (ratio 1:2), tri-silicates or 
poly-silicates (ratio 1:3 or higher), etc. All the bases are not known to combine 
in all these ways, but many do in more than one. 

If the silica united with each of the bases one by one, the results would still 
remain comparatively simple; but instead, it may unite with two or more at the 
same time. Thus we may have an aluminum-calcium silicate. Not only this, 
but the different silicates may crystallize together in the same mineral, so that a 
crystal may be made up of alternating layers of different silicates. As such 
alternations are not governed by any known mathematical law, there is no deter- 
minate limit to the number of combinations that may arise. 

As a result of all this fertility of combination, the total number of siliceous 
minerals in igneous rocks is large. Geology deals with these minerals as constit- 
uents of the earth, but only a few of them are so abundant as to require special 
notice here. It may be remarked also that, as they occur in the rocks, only a few 
of them can be identified by simple inspection, partly because some of them look 
much alike, and partly because many of the crystals are minute. 


Summary of salient facts. The salient facts are, (1) that out of 
the 70-odd chemical elements now known in the earth, eight form 
the chief part of it; (2) that one of these elements uniting with the 
rest forms nine leading oxides; (3) that one of these oxides acts as 
an acid and the rest as bases; (4) that by their combination they 
form a series of silicates of which a few are easily chief; (5) that 
these silicates crystallize into a multitude of minerals of which again 
a few are chief; and (6) that these minerals are aggregated in various 
ways to form rocks. Possessed of these leading ideas, we are pre- 
pared to turn to the consideration of some of the conditions under 
which these combinations take place in the formation of rocks from 
liquid magmas. 

Principal minerals of igneous rocks. A few minerals make up 
the mass of igneous rocks. ‘These few are quariz, the feldspars, the 
ferro-magnesian minerals (amphiboles, pyroxenes, micas), and the 


254 MATERIALS AND THEIR ARRANGEMENT 


iron oxides. ‘These minerals are described briefly below. Some of 
them occur in sedimentary and metamorphic works, as well as in 
igneous rocks. 


Quartz. SiOe; H. 7; Sp. gr. 2.65. A mineral of very widespread occurrence. 
It is found not only in igneous rocks, but in veins and cavities in other sorts of rock, 
as nodules and concretions-in limestones, and is the most abundant constituent 
of sands, sandstones, and quartzites. 

Quartz is a very hard mineral; that is, it cannot be scratched with steel and it 
will scratch glass. It is said to have conchoidal fracture; that is, it breaks like 
glass, without any distinct tendency to break along parallel planes. In igneous 
rocks, quartz usually looks rather dark and glassy by contrast with the lighter 
colored, less transparent minerals with which it is commonly associated. Some 
quartz has a sort of greasy or oily look because of its comparatively high luster. 
In veins and cavity fillings it may occur, (1) as 6-sided crystals capped by pyra- 
mid-like forms. Thecrystals may be so closely spaced that only the pyramid-like 
forms can be seen; (2) as a sort of hummocky crust with a waxy luster (chalcedony), 
or (3) as a series of bands of variegated color (agate). As concretions in limestone 
it may have a variety of colors, but is usually between white and dark grey, in some 
cases nearly black. Concretions are usually irregular in form, and contain a 
considerable proportion of impurities, but can be recognized by the hardness of a 
freshly broken surface. 

Some of the less common varieties are used as semi-precious stones; for ex- 
ample, amethyst, cairngorm, rose quartz, jasper, prase, cat’s-eye, and agate. True 
onyx is also a variety of quartz. 

The feldspars. H. 6; Sp. gr. 2.5-2.6. 

Orthoclase, Potassium aluminum silicate 

: Sodium aluminum silicate 

Plagioclase ie lei ; sae 

alcium aluminum silicate 

Feldspars are abundant in igneous rocks and their metamorphic products, 
but are’not found abundantly in other rocks. Feldspars are not so hard as quartz, 
but cannot be scratched by any but the very hardest steel. They have good 
cleavage; that is, they have a strong tendency to break along parallel plane sur- 
faces. This can be detected by holding a freshly broken surface to the light so 
that a reflection is seen. If the whole surface of a crystal seems to reflect the light 
when the fragment is held in a given position, it is usually due to the cleavage of 
the mineral. Feldspars are commonly the dominant light colored constituents of 
igneous rocks, but they range in color through white, buff, pink, red, and grey, 
and a comparatively rare variety is green. 

It isnot always easy to distinguish orthoclase, KAISi3;Os, from plagioclase, a mix- 
ture of NaAISi30g and CaAl,Si.0g; but the cleavage faces of some plagioclase crys- 
tals show distinct parallel striations, almost as true as if made by a ruling engine. 
These are never present in orthoclase. The dark, dull or waxy looking feldspars 
are more likely to be plagioclase, while buff or pink feldspars are more likely to 
be orthoclase; but such distinctions are too uncertain to be used with great assur- 
ance. 

Some of the rarer feldspars are used as semi-precious stones. Among these 
are Amazonstone, sunstone, moonstone, peristerite and labradorite. . 


2 — 


MINERALS OF IGNEOUS ROCKS 255 


Amphiboles and pyroxenes. Complex silicates, usually containing iron, lime, 
and magnesia. H. 5-6; Sp. gr. 2.8-3.6. The amphiboles and pyroxenes occur 
chiefly in igneous and metamorphic rocks, in some of which they are the most 
abundant dark colored constituents. 

They are hard minerals, that is, can be scratched by steel with difficulty. 
The commoner ones are black, greenish black or brown. MHornblende is the most 
important of the amphiboles, and azgite of the pyroxenes. These minerals re- 
semble eachother rather closely, and in very small crystals it is in some cases diffi- 
cult to distinguish them. The most notable difference is in the cleavage. Horn- 
blende cleaves in two directions at an angle of 124° (and 56°) from each other, while 
in augite the two cleavage directions are nearly perpendicular to eachother. 
Most hornblende has a jet-like luster, while augite is more likely to be dull, and 
is likely to be coated with rust on weathered surfaces. Another amphibole of 
importance is actinolite, which occurs only in metamorphic rocks and is easily 
recognized by its long, slender, needle-like crystals, which have a diamond-shaped 
cross-section and are usually bright green in color. With the exception of bronzite, 
which is distinguished by its brown color, the other important pyroxenes cannot 
be distinguished readily from augite. 

The micas. Complex hydrated silicates. H. 2; Sp. gr. 2.76-3. The common 
micas occur in igneous and metamorphic rocks and to a small extent as minute 
flakes in some sandstones and shales. The commonest micas are muscovite, which 
is white, greenish or yellowish brown, and biotite, which is dark brown or black. 
They are very soft, that is they can be scratched with the thumb-nail, and have a 
conspicuously good cleavage — so good that they may be split into sheets thinner 
than the thinnest paper. These thin sheets or flakes are very elastic and tough. 
In metamorphic rocks the plates are roughly parallel, and this results in the char- 
acteristic schistose appearance of many such rocks. Microscopic flakes of mica, 
arranged parallel to one another are responsible for the cleavage of slates. The 
only simple means of distinguishing between muscovite and biotite is the color. | 

Olivine. (FeMg)2SiOu; H. 6.5—7; Sp. gr. 3.3-3.5. Olivine occurs in the more 
basic igneous rocks, that is, those that are comparatively low in silica, especially 
those that are rich in ferromagnesian minerals. Olivine also occurs in meta- 
morphic rocks, especially metamorphosed dolomitic limestones. It is a very hard 
mineral, and has conchoidal fracture. In color it is yellowish green, varying 
somewhat according to the amount of iron present. It is usually transparent, and 
has a vitreous luster. It commonly occurs in granular aggregates which contain 
some pyroxene, but is found also in crystals in some dark colored igneous rocks. 
A variety used as a gem is called peridot. Chrysolite is another name applied to 
olivine. | 
The iron oxides. 

Hematite, FexO3; H. 5.5-6.5; Sp. gr. 4.8-5.4. 

Limonite, 2Fe203,3H2O; H. 5-5.5; Sp. gr. 3.6-4. 

Magnetite, Fe3;04; H. 6; Sp. gr. 5.18. 

Hematite + is the most important of the iron ores. It occurs (1) in sedimentary 
rocks, in some cases being the cementing material in sandstones, as in many of the 
red sandstones; (2) in igneous rocks as the result of weathering of the iron minerals; 
(3) in veins, and (4) in contact metamorphic deposits. It is mostly hard, but 


1 Turgite is here included with hematite. _ 


256 MATERIALS AND THEIR ARRANGEMENT 


shows some variation in hardness, as some specimens can be scratched by steel 
while others cannot. Hematite is red, dull steel blue, or black. Its most char- 
acteristic feature is the color of the streak, that is, the color of the fine powder left 
behind when the specimen is drawn across an unglazed porcelain plate, or powdered 
very finely inany way. ‘The color of the streak or fine powder is dark brownish red. 
The pigment called Venetian red is merely very finely ground hematite, contain- 
ing a small amount of clay. 

Limonite occurs in the same sorts of situations as hematite and as an alteration 
product of many ore deposits containing iron sulphides. It is deposited in some 
bogs in sufficient quantity to be valuable as an iron ore. Chemically it is the 
same as iron rust. Limonite has a very wide range of hardness, from soft earthy 
material, to compact material which cannot be scratched with steel. In color 
it ranges from light yellowish brown to very dark brown — in some cases almost 
black. The streak is characteristically yellowish brown, no matter what color 
the specimen has in mass. The common pigment yellow ocher owes its color to 
limonite. 

Magnetite. Magnetite occurs in igneous and metamorphic rocks, in contact 
metamorphic deposits, and to a limited extent in veins. It is very hard, and 
characteristically black in color, except on weathered surfaces, where it is usually 
coated with rust. The streak is black. Magnetite is most easily recognized by 
its magnetic properties. It is strongly attracted by a magnet, and may be mag- 
-netized. Naturally magnetized magnetite is called lodestone, and it is from this 
substance that our term magnet is derived. Magnetite is of comparatively little 
importance in America as an iron ore, but in some parts of Europe it is a very 
important ore mineral. 

Other important rock-forming minerals. A few other common rock-making 
minerals are mentioned here, though most of them do not occur in igneous rocks, 
except as secondary minerals introduced subsequent to the hardening of the lava. 
Several of them occur in metamorphic rocks only. Calcite and dolomite occur 
abundantly in certain sedimentary rocks, but are secondary in igneous rocks. 

Calcite. Calcium carbonate, CaCO3. H. 3; Sp. gr. 2.72. Calcite is a mineral 
of very widespread occurrence. It is the chief constituent of limestones and 
marbles (metamorphosed limestones), and occurs as cavity fillings in many kinds 
of rocks. It is a very common vein mineral and occurs as an alteration product 
of lime silicates in many weathered igneous rocks. Calcite is rather soft; that is, 
while it cannot be scratched with the thumb-nail, it is scratched easily with steel. 
It has a very good cleavage in three directions which may give rise to rhombohe- 
drons, that is figures like cubes, which have been compressed along one diagonal. 
It is recognized most readily by its behavior towards acids, which act upon it 
rapidly, causing an effervescence of carbon dioxide. Some other minerals effer- 
vesce similarly, but none of the very common ones show such rapid action as 
calcite. Transparent pieces of calcite show double refraction; that is, if a piece of 
transparent calcite is placed over a dot on a piece of paper, two dots may be seen 
distinctly. Most calcite is white or colorless, but some is colored brown, yellow, 
green or pink by impurities of various sorts. 

Chlorite. A complex, hydrated silicate containing Fe and Mg. H. 2; Sp. gr 
2.65-2.75. Chlorite is a secondary mica; that is, it is a mica formed by the action 
of weathering on certain silicates containing iron and magnesium. It occurs in 
metamorphic rocks, and as an alteration product in igneous rocks, It is very 


MINERALS 254 


soft, usually dark green or greyish green in mass, and gives a grey or greenish grey 
streak. Chlorite has micaceous cleavage, and the plates are mostly flexible and 
inelastic. The greenish color of many igneous rocks is due to the presence of 
chlorite formed by the alteration of pyroxenes, amphiboles, etc. 

Dolomite. CaMg(COs3)2. H. 3.5-4; Sp. gr. 2.8. A mineral closely resembling 
calcite. Dolomite occurs in some of the older limestones, in some marbles, and to 
a small extent in veins. It is a rather soft mineral, showing a cleavage like calcite 
when crystals of sufficient size are seen. Some dolomitic rocks are so fine grained 
that the individual crystals cannot be detected without the aid of the microscope, 
and in such specimens no cleavage is apparent. Some dolomite crystals exhibit 
peculiarly curved faces, which are rare in calcite. Most dolomite is milky white, 
brownish, or pink, but it varies greatly in color with impurities of various sorts. 
It is most easily distinguished from calcite by its behavior with dilute acids. 
Calcite effervesces vigorously even in quite dilute acids, while dolomite is only 
very feebly attacked by such acids. 

Garnet. A complex silicate, different varieties having different composition. 
16-5-7.5;9D. gr. 3.15—4.3. 

Garnet is rarely found outside of metamorphic rocks, in which it is in some 
instances so abundant as to be the chief constituent. It is a very hard mineral — 
about as hard as quartz; but when it has been exposed to weathering for a long 
time it may in some cases be scratched with steel. Garnet has no cleavage, but 
breaks with conchoidal fracture like glass or quartz. In color most garnets are 
red or brown, but other colors, as pink and green, are known. Garnet has rather 
high specific gravity compared with the rest of the common silicates. Garnet 
is used in large amounts as an abrasive, and to a small extent as a gem stone. 

Graphite (carbon). H. 1-2. Sp. gr. 2.1. 

Graphite is one of the crystalline modifications of carbon. It occurs in meta- 
morphic rocks — mostly in metamorphosed sedimentary rocks. It is very soft 
and has a marked greasy feel, like that of talc. It is black, and will mark paper 
with a black streak, a property of which use is made in common lead pencils. 
Graphite also is used in the manufacture of crucibles, as a lubricant, and as an 
adulterant. 

Gypsum. Hydrated calcium sulphate; CaSO4,2H2O; H. 2; Sp. gr. 2.3. 

Gypsum occurs in beds among other sedimentary rocks, in some places as a 
residue from the evaporation of saline lakes, and in crystals scattered through 
shales. It occurs sparingly in veins. It is a very soft mineral and commonly 
has a very good cleavage, resembling mica, except that the thin leaves are not 
elastic nor strong like those of mica. It has not the greasy feel characteristic of 
talc. A number of varieties of gypsum are recognized. Selenite, the common 
variety of gypsum, has been described above. Satin spar is a variety that has a 
fibrous structure, and a bright satin-like luster. Alabaster is white massive gypsum; 
that is, gypsum which is made up of minute crystals which cannot be readily dis- 
tinguished as individual crystals. Gypsum is characteristically white, but may be 
reddish, brownish or gray when it is impure. Gypsum is largely used in the 
manufacture of various sorts of plaster. Alabaster of high grade is sometimes 
used for ornamental vases, etc. 

Kaolin. A hydrous aluminum silicate. H. 2-2%; Sp. gr. 2.6. Kaolin 
occurs in igneous rocks as an alteration produce of feldspars, and in sedimentary 
rocks. Shales and clays are made up largely of kaolin, and it is present in varying 


258 MATERIALS AND THEIR ARRANGEMENT 


quantities in other sedimentary rocks. It is a very soft mineral, and when free 
from grit usually has a soapy feel, but differs from talc in that it becomes plastic 
if ground up and moistened with water. Kaolin is earthy in appearance, and 
breaks like an earthy substance. No cleavage is apparent, because the mineral 
is not commonly crystallized, and when crystallized the individual crystals are too 
small to show cleavage without the aid of the microscope. Its color is nearly 
white when pure, but more commonly is brown, or bluish gray, according to the 
impurities it contains. 

Pyrite. FeSy. H. 6-6.5; Sp. gr. 5.0. Pyrite occurs in minute crystals in 
igneous rocks, in large masses in some veins and in metamorphic rocks, and is 
not uncommon in limestones, sandstones and shales. It is abundant in some 
coal beds and is the source of the sulphurous odor of coal smoke. Pyrite is very 
hard, has a bright metallic luster resembling that of light colored brass, but its 
streak is black. Crystals of pyrite may be cubes, octahedrons, or more complex 
forms; slightly deformed cubes with striated faces are the most common. When 
exposed to weathering, pyrite rusts — that is changes to limonite. If the original 
form of the pyrite crystal is retained by the limonite, it is called a pseudomorph. 
Pyrite is used in the manufacture of sulphuric acid, and to a very small extent as 
a source of low-grade iron. 

Serpentine. Hydrated silicate of magnesium. H. 4; Sp. gr. 2.5-2.6. Ser- 
pentine occurs chiefly in metamorphic rocks, and as an alteration product in basic 
igneous rocks. Serpentine has a greasy feel, but less marked than that of talc. 
Most of it is yellowish green or yellow in color, but may be stained brown by 
iron oxides. One variety of serpentine consists of fine, closely packed, flexible 
fibers, called asbestos. Most asbestos occurs in vein-like masses in massive 
serpentine, the fibers running across the vein. Serpentine is used as a building 
stone and for interior decoration; asbestos is used in fire-proofing and in the thermal- 
insulation of material of various sorts. 

Talc. A hydrous magnesium silicate. H. 1; Sp. gr. 2.75. Talc is an alteration 
product of magnesium silicates, especially those free from alumina, and abounds in 
their metamorphic products. It may occur about the mineral grains in weathered 
igneous rock, but occurs more abundantly in metamorphic rocks, such as soap- 
stone and talc schist. It is very soft and has a peculiar greasy feel that 
usually serves as a valuable aid in identification. Some specimens show very 
good cleavage, resembling mica except for the fact that the thin plates are 
not elastic. In translucent specimens most talc is light green in color; in 
opaque specimens, it varies from nearly white to dark gray. Some varieties 
resemble kaolin (pure clay), but may be distinguished readily from it by moisten- 
ing some of the finely powdered mineral; kaolin becomes plastic while talc does 
not. 

Classification of Igneous Rocks 

Several features are involved in the classification of igneous rocks. Some 
of them have been noted already, but may be recapitulated here. All fragmental 
igneous rocks are pyroclastic, and pyroclastic rocks may be tuffs, agglomerates, etc. 
(p. 263). Rock formed from lava without the development of crystals, is obsidian, 
if not porous. If porous (hardened rock-froth), the rock is pumice, scoriaceous 
glass, etc. If the rock is largely glass, but partly of small crystals, it is sometimes 
called pitchstone, because its freshly fractured surface looks like pitch or resin. 
When the cavities of scoriaceous rock become filled by minerals deposited from 


~ 


ee 


CLASSIFICATION OF IGNEOUS ROCKS 259 





Fig. 240. Graphic granitic (or pegmatitic) texture. Nearly natural size. 
(Photo. by Church.) 


solution, the rock becomes an amygdaloid. Porphyry, phanerite and aphanite 
have been defined already (p. 248). All these names are based on texture, rather 
than on mineralogical or chemical composition, 

Most igneous rocks are wholly crystalline, and are classified on the basis of 
their composition. Their chemical composition determines their mineral com- 
position, and the rocks are named according to the minerals they contain. The 
number of varieties of igneous rock is very large, but only a few of the more im- 
portant need be’ mentioned here. 

The granites. The name granite was originally used to designate a granular 
rock (a phanerite, p. 248), and it is still popularly and properly so used. In scientific 
treatises it usually has been confined to a rock composed chiefly of crystals of 
quartz, feldspar (especially orthoclase) (p. 254), and mica. Recently it has been 
proposed to give it again a more general application by including under it all 
phanerites composed chiefly of quartz and feldspar of any kind, with mica, 
hornblende, or other minerals in subordinate amount. In normal granite, the 
crystals are distinct and in some cases large (Fig. 237), and more or less intimately 
interlocked. Granites are among the most common and easily recognized of the 
phanerites. Their color is determined largely by the feldspar, the red and pink 
varieties of the mineral giving rise to red and pink granite, and the whitish varie- 
ties to gray granite. Granites vary widely from their type by the addition and 
substitution of other minerals. Whenever one of these replacing or accessory 


260 MATERIALS AND THEIR ARRANGEMENT 


minerals, is abundant, its name is often prefixed, as hornblende-granite. Granite 
grades insensibly into other types of igneous rock, as syenite, diorite, etc. Varia- 
tions also arise from the absence of one. of the leading minerals. 

Granites were formed from lavas rich in silica (normally 68-70%), alumina, 
potash, and soda, but generally poor in lime, iron, and magnesia. Granite is 
generally an intrusive massive rock. When rock of the composition of granite is 
banded, it is gneiss. 

Graphic granite, composed chiefly of inter-grown crystals of quartz and feld- 
spar, has a peculiar texture (Fig. 240). Pegmatite is a variety of coarsely cry- 
stalline granite composed chiefly of quartz, feldspar, and muscovite (p. 255). It 
occurs principally in dikes and veins associated with granitic and other similar 
rocks. Rock of similar texture may have the composition of syenite (syenite 
pegmatite), diorite (diorite pegmatite) etc. 

The syenites. The term syenite (from Syene on the Nile, where this sort 
of rock occurs) is now applied to rock consisting essentially of feldspar and horn- 
blende, with or without mica; but there is a complete gradation from granites to 
syenites. Syenites are richer in iron and magnesium than granites, and poorer in 
silica (about 58-60%). Syenites also grade into other classes of rock as do granites, 
and special varieties are named by similar prefixes, as augite-syenite, etc. Syenites 
are red or gray according to the color of the feldspar, and most of them are rather 
darker than granite, which they resemble. The texture of syenite is like that of 
granite, and its mode of occurrence the same. 

The diorites. Diorites are rocks which crystallized from lavas having about 
the same amount of silica as the lavas of the syenites, but poorer in the alkalies, 
and richer in the earthy bases. In current usage, diorite is defined as a rock 
composed of an intimate mixture of crystals of hornblende and a plagioclase feld- 
spar. It differs from syenite in having plagioclase feldspar (p. 254) instead of 
orthoclase. By substitutions and the addition of accessory minerals, the diorites 
grade toward the granites and syenites on the one hand, and toward the gabbros 
on the other. In color most diorites are rather darker than the gray granites. 

The gabbros. The gabbros embrace a large group of rocks whose principal 
minerals are plagioclase (normally labradorite) and pyroxene (normally diallage), 
with magnetite or ilmenite (titanium iron oxide). Most gabbros are dark colored 
and rather heavy. The pearly luster of the cleavage faces of the diallage gives a 
peculiar sheen to a fresh surface of the rock, in many cases. Silica constitutes 
about 46 to 55 per cent of gabbros. 

The peridotites. Peridotites were formed from a magma in which silica was 
low (39-45%), as were also alumina, lime, and the alkalies, but in which magnesia 
was high (35-48%). The rock consists largely of the mineral olivine (p. 255.) asso- 
ciated with pyroxene, magnetite, and other basic minerals. Little or no feldspar 
is present. Peridotites are much less abundant than the preceding rocks. 

Closely allied with the peridotites are rocks made up largely of a single basic 
mineral, as augitite, pyroxenite, hornblendite, rocks essentially formed of the min- 
erals augite, pyroxene, and hornblende, respectively. 

The basalts. The term basalt is used in a somewhat comprehensive way for 
dark, compact, igneous rocks the crystals of which are in most cases so minute as 
not to be distinguished readily by the eye. The leading minerals are a plagioclase 
feldspar and pyroxene (usually augite), with olivine, and magnetite or ilmenite 
usually present. There is a considerable range in chemical composition, but the 


CLASSIFICATION OF IGNEOUS ROCKS 261 


basalts are relatively poor in silica (46-55%), and most of them in potash and sada, 
but rich in lime, magnesia, and iron. Basalts are classed as basic, and some are 
highly so. The lavas of many basaltic flows were very fluid, and spread out.in 
thin sheets when poured‘ out upon the surface. In cooling, basalt is prone to 
take on a columnar structure (p. 248). The columns of Giant’s Causeway and Fin- 
gal’s Cave are familiar examples. 

Basalts graduate insensibly into dolerites, which may be regarded as basalts of 
coarse crystallization. Diabase is a rock of similar composition and ophitic texture; 
that is the pyroxene crystals are separated into thin plates by inter-growths of 
plagioclase. 

General names. The difficulty of distinguishing many of the foregoing rocks 
from each other by any means available in the field, owing to the minuteness of 
the crystals, and to the gradation of one type of rock into another, makes it desir- 
able to employ certain general names which will correctly express the leading 
character of the rock without implying a knowledge of its precise mineral com- 
position. A convenient term of this kind is greenstone, which merely indicates 
that the ferro-magnesian minerals are prominent, and give a greenish or greenish 
black cast to the rock. The greenstones embrace the dolerites and basalts, and 
some of the gabbros and diorites, and may even extend to the peridotites and per- 
haps to others. Another convenient name is trap, which may be used for any 
dark, heavy igneous rock, such as basalt. The term basalt is sometimes used in 
much the same way. 

Varieties of rock dependent upon conditions. From what has preceded, it is 
clear that the chemical nature of the liquid magma determines the mineralogical 
composition of the rock, if it is crystalline; but it may now be pointed out that 
the same lava which made a plutonic granite, might have made a porphyry, an 
obsidian, a pumice, or a tuff, under other conditions of solidification. The same is 
true of diorites, gabbros, etc. 


A New System of Classification and Nomenclature of Igneous Rocks 


The current systems of classifying and naming rocks, if indeed they can be 
called systems, have grown up gradually out of earlier and cruder methods, many 
of which were inherited from popular usage. Most of the names and definitions 
came into use before modern methods of study were adopted. These systems, 
therefore, retain many crudities and inconsistencies, and lack adaptation to present 
needs and knowledge. A more adaptive and consistent classification is needed, 
and in response to this need, a new system of classification of igneous rocks has 
been offered by a group of leading American petrologists.1 To some extent this 
proposed system may be extended to metamorphic crystalline rocks. The classi- 
fication and nomenclature of the'sedimentary rocks probably must always remain 
plastic, to express the various points of view which it is desirable to consider. 

The proposed system includes two parts, a field system and a quantitative 
system, the one applicable to rocks on casual inspection, and the other only after 
detailed study. The field system only is here outlined. 

The proposed field system. ‘The proposed field names are based largely on 
texture and color. Mineral constituents are used for subdivisions when they can 
be determined easily; otherwise they are neglected. 


1 Cross, Iddings, Pirsson, and Washington. Quantitative Classification of 
Igneous Rocks. See also Johannsen, Jour. Geol. Vol. XIX (1912), p. 317. 


262 MATERIALS AND THEIR ARRANGEMENT 


Classifying chiefly on the basis of texture and crystallinity, there are three 
groups: Phanerites, in which all the leading mineral constituents can be seen with- 
out a lens; aphanites, in which at least an appreciable part of the minerals cannot 
be distinguished by the unaided eye; and glasses, in which the material is wholly 
or largely vitreous. 

I. The phanerites are classified further as follows: 

1. Granites, consisting largely of quartz and feldspar of any kind, with or 
without mica, hornblende, pyroxene, or other minerals. This differs from the 
present common use of the term granite, in not regarding mica as an essential 
constituent, and in not distinguishing between alkali feldspars and calcic feldspars. 
The term therefore includes more than formerly. 

2. Syenites, consisting predominantly of feldspar of any kind, with subordinate 
amounts of hornblende, mica, or pyroxene, but with little or no quartz. This 
differs from the common usage in giving hornblende a subordinate place, and in 
embracing rocks with calcic feldspars. 

3. Diorites, consisting predominantly of hornblende and subordinately of 
feldspar of any kind, with which there may be mica, pyroxene, or other minerals. 
This is nearly the present use, except that any kind of feldspar may be the sub- 
ordinate mineral. 

4. Gabbros, consisting predominantly of pyroxene and subordinately of feld- 
spar of any kind, with or without other minerals. This nearly coincides with one 
of the various present uses of the term, except that the range of the feldspar is 
increased. 

5. Dolerites,: consisting predominantly of any ferromagnesian mineral not 
distinguishable as hornblende or pyroxene, with subordinate amounts of feldspar 
of any kind, and with or without other accessory minerals. In other words, the 
dolerites (deceptive) embrace diorites and gabbros when they cannot be distin- 
guished by the eye. 

6. Peridotites, consisting predominantly of olivine and ferromagnesian minerals 
without feldspar, or with very little. 

7. Pyroxenites, consisting essentially of pyroxene. 

8. Hornblendites, consisting essentially of hornblende. 

Lh. bhe aphanites may be non-porphyritic or porphyritic. 

(a) Non-porphyritic aphanites, if light-colored, may be classed as felsites: 
when dark-colored, as basalts. 

(b) The porphyritic aphanites or porphyries, if light-colored, are /eucophyres, 
when dark-colored, melaphyres. They may be classified further, according to the 
kind of phenocryst (distinct crystal) imbedded in the aphanitic groundmass, as 

Quartz-por phyries, or quartzophyres; 

Feldspar-por phyries, or felspaphyres (not felsophyres) ; 

Hornblende-por phyries, or hornblendophyres; and so on. 

These may be subclassed by color, as 

Quartz-leuco phyres, light-colored quartz-porphyries; 

Quartz-melaphyres, dark-colored quartz-porphyries; 

Feldspar-leucophyres; 

Feldspar-melaphyres; and so on. 

III. The glasses are classified, according to color and luster, into obsidians 


1 Added by the authors of this work. 


DISRUPTION OF IGNEOUS ROCKS ele 


or pitchstones when dark and lustrous; perlites when a spheroidal fracture gives 
them a pearly appearance; and pumice when greatly inflated by included gases. 

IV. Pyroclastic rocks are 

Tuff, if composed of finely comminuted pyroclastic material; 

Volcanic breccia if composed of coarse angular pyroclastic materials, Agglom- 
erate is a term much used for volcanic breccia, and for similar rock whose con- 
stituents are but little rounded. If the constituents are well rounded, the rock 
becomes volcanic conglomerate. 

In general discussions, it is serviceable to use the term granitoids in a broad 
generic sense, to include all crystalline rocks of the general granitoid type, including 
the granites, syenites, etc. In a similar broad way, gabbroids may be used to 
include the dark crystalline rocks in which the ferromagnesian minerals predomi- 
nate, as the diorites, gabbros, dolerites, peridotites, etc. In this convenient and 
comprehensive way, two contrasted groups of igneous rocks may be designated. 
As the granitoids are usually acidic and the gabbroids basic, the grouping repre- 
sents a broad fact of importance. 


The Disruption of Igneous Rocks 


At the surface, igneous rocks are subject to mechanical dis- 
ruption, and to chemical change which results in decay. 

Mechanical disruption. One great agent of mechanical disrup- 
tion at the surface is change of temperature. This has been dis- 
cussed in Chapter II and other phases of mechanical disruption 
are discussed in Chapters IV and V. All mechanical disruption of 
igneous rock leaves the fragments essentially like the original rock 
in composition. 

Chemical disintegration. Most of the silicate minerals which 
make up the larger part of all igneous rock are complex, chemi- 
cally. Not a few of them contain as many as three or four basic 
elements, in union with oxygen and silicon. Substances which are 
complex chemically, are, as a rule, less stable than those of simple 
constitution. Complex silicates, such as the feldspars, micas, 
amphiboles, and pyroxenes tend to break up into simpler sub- 
stances. Chemical changes are helped along by the oxygen, carbon 
dioxide (COz), and water vapor of the air, and by water after it is 
precipitated. Some of the simpler changes may be noted. 

Oxygen may enter into combination with the iron of a silicate 
mineral containing iron. ‘The iron is thus taken out of its silicate 
combination, and in union with the oxygen forms iron oxide, a 
simple and stable chemical compound. This process is oxidation. 
Oxidation affects other elements also. 

Similarly, carbonic dioxide from the air may enter into com- 
bination with the base of a silicate mineral. Thus it enters into 


264 MATERIALS AND THEIR ARRANGEMENT 


combination with the calcium of a mineral which contains calcium, 
taking the latter out of its union with silica. The union of the 
calcium and the carbon dioxide gives rise to calcium carbonate. 
Magnesium and iron may be taken out in the same way, forming. 
magnesium carbonate and iron carbonate, respectively. This proc- 
ess is called carbonation, and the carbonates thus formed are simple 





Fig. 241. Exfoliation of granite. Wichita Mountains, Okla. 


and stable in composition. ‘The carbonates are more soluble than 
most other common mineral substances. 

Water may enter into combination with mineral matter, and the 
union is hydration. ‘Thus when iron rusts (oxidizes), it is not 
merely oxygen which enters into combination with the iron, but 
water also. Iron rustis a hydrated oxide of iron (see limonite, p. 256). 

Oxidation, carbonation, and hydration, involving respectively 
the addition of oxygen, carbon dioxide, and water, increase the 
volume of the mineral matter. The result is that the rock affected 
crumbles. Thus the iron rust formed on a knife blade crumbles 
off. So the iron rust formed when oxygen and water unite with 
the iron in the rock, causes the rock in which the change takes 
place to crumble, partly because of the expansion involved. 

Again, some of the simple compounds, especially the carbonates, 
formed when the rock decays, are somewhat soluble and may be 
dissolved and taken away. ‘This tends to make the rock less com- 
pact by taking away one of its ingredients. 

Oxidation, carbonation, and hydration therefore. not only 


SEDIMENTARY ROCKS 265 


change the chemical nature of the rock, but they change its volume, 
allow some of its material to be carried off in solution, and in many 
cases cause it to fall to pieces. The result is decayed rock — or 
one variety of rock waste. It is to be observed that the rock waste 
which arises from decay is unlike the original rock in composition. 





Fig. 242. Exfoliation on the slope of a granite mountain near Royal Arch Lake, 
Yosemite Quadrangle. (Turner, U.S. Geol. Surv.) 

Some things have been added, and others taken away. In this 
respect, the waste arising from decay is unlike that arising from rock 
breaking. 

The products of decay may remain where formed, or may be 
taken away. If they remain where formed for long periods of time, 
they may come to make a thick mantle of residual earth. Decayed 
rock is scores of feet in depth in many places, and hundreds of feet 
in some. Chemical decomposition is greatest in warm regions, and 
products of decay are least readily removed where there are forests. 
The products of decay are therefore likely to be deepest in warm, 
forested regions. ‘They are very deep, for example, in some parts 
of Brazil. 

SEDIMENTATION AND SEDIMENTARY ROCKS 


Removal of decayed rock. The breaking-up of igneous rock 
prepares the way for other processes. The loosened material may 
be blown away by the wind, washed away by running water, or 


266 MATERIALS AND THEIR ARRANGEMENT 


moved by any agency which shifts materials about on the surface 
of the earth. If the products of rock disintegration are coarse, they 
may become gravel after being rounded by streams or waves. If 
the material is finer, say of the size of small grains, it is sand; if 
still finer, it is mud when wet, and.dust when dry. 

Deposition of sediment. When carried by any transporting 
agency, such as wind or water, rock waste becomes sediment, and 
sooner or later is deposited. Some of the material picked up and 
transported by running water is left at the bases of the slopes of 
mountains and hills from which it is washed, and some of it is 
left on the flats through which streams flow; but much of it is 
carried to the sea and left there. The coarser part of the sediment 
carried to the sea is left near the shore, and the finer parts are taken 
farther out. Thus along many coasts the gravel of the shore-line 
grades out into sand, and this into mud as distance from the water’s 
edge increases. The coarser materials are thus separated more or 
less perfectly from the finer. 

When the disintegration of the parent rock results from decay, the 
rock-waste is unlike the parent rock in composition, because some of 
the original material has been dissolved and carried away in solution. 
Not only this, but the fine products of decay may differ from the 
coarser in composition. Thus the quartz grains of granite are 
generally large enough to be readily seen individually; and as the 
granite decays, this mineral, already a simple compound, undergoes 
little change, and the grains remain in the rock waste. By moving 
water, they are rounded into the sand grains with which we are 
familiar. On the other hand, the crystals of feldspar, which have a 
complex composition, decompose into very fine particles of kaolin 
(p. 257) or clay, unlike the feldspar in composition, and containing 
but a few of the elements of feldspar. Thus it happens that the 
coarser products of decay, such as quartz, are chemically unlike 
the finer, such as clay, and the two are partly separated when they 
are deposited. In this case, the composition of the rocks formed 
from the sediments may be very different from that of the rock 
from which the sediments were derived. On the other hand, when 
rock-waste resulting from the mechanical breaking of rock is depos- 
ited, the sediment has about the same composition as the rock from 
which it came. Sediment which contains feldspar derived from 
granitic rock is called arkose. Arkose represents incomplete decom- 
position of the parent rock. 


SEDIMENTARY ROCKS 267 


Cementation of sediment into solid rock. After gravel, sand, 
mud, etc., are deposited in the sea or elsewhere, they may be cement- 
ed into solid rock by the deposition of mineral matter held in solu- 
tion in water. This cement binds the pebbles, the grains, and the 
smaller particles together, much as lime binds sand in mortar. The 
cemented gravel makes conglomerate, or if the pieces of rock are 
angular, breccia; the cemented sand makes sandstone; and the ce- 
mented mud makes shale. These are common sorts of sedimentary 
rock. The cementation may take place while sedimentation is in 
progress, or at a later time. Conglomerate, sandstone, and shale, 
made up chiefly of particles derived directly from other rock, are 
clastic rocks. Limestone may be broken up, and its particles 
redeposited and cemented again into solid rock. Such limestone 
is clastic, and limestone made of broken shells, coral, etc., is in some 
sense clastic. In contrast with igneous rocks, clastic rocks are 
made up of particles of other rock, particles which were once separate 
and distinct, bound together by some sort of cement. The particles 
touch one another, but do not interlock like crystals of igneous rock. 

When sand, mud, etc., are deposited in the sea, shells of sea 
_ animals may be imbedded in them. If the shells or their forms are 
preserved, they record the kinds of life that lived when the sediment 
was being laid down. If the sediments are deposited in lakes or on 
land, the shells or other relics of freshwater or land life may be 
buried in them. All distinct relics of past life are fossils. 

Non-clastic sediments. Not all sedimentary rocks are clastic. 
It has already been noted that some of the compounds formed when 
rock decays are soluble. A part of the materials dissolved are 
carried in solution to the sea, where some of them are extracted by 
animals and made into shells or other hard parts. When the 
animals die, their shells and other secretions are left behind. If 
these are of calcium carbonate, they make limestone when cemented 
together. Much, if not most, limestone is composed of the secre- 
‘tions of organisms. 

The shells, coral, etc., may or may not have been broken up 
before cementation. Limestone has many varieties, one of which is 
chalk. Magnesium may replace the calcium in various propor- 
tions, and if there is any considerable amount of magnesium, the 
rock is dolomite. ‘The dolomization of some limestone (the con- 
version of CaCO; into CaMg(COs)2) appears to have taken place 
long after the limestone was formed, while in other (perhaps in 


268 MATERIALS AND THEIR ARRANGEMENT 


most) cases it appears to have taken place while the material of 
the limestone was being deposited. 

Siliceous deposits. In the decomposition of igneous rocks, a 
little of the silica, as well as of the bases, is dissolved and carried 
away in solution. Certain organisms extract this silica from the 
water for their tests, shells, etc., just as others extract calcium car- 
bonate. Siliceous secretions may form siliceous rocks. Dziatom and 
radiolarian oozes of the deep sea are examples. Familiar examples 





Fig. 243. Globigerina ooze, similar to chalk in composition. Magnified 20 
times. (Murray and Renard.) 


of indurated rock formed in this way are certain flints and cherts 
that occur in limestone, both as nodules and in distinct beds. 
Some of these are developed about fossil sponges. 

Precipitation from solution. Some sedimentary rock is formed 
by direct precipitation from water which is saturated. Thus 
limestone might be formed by direct precipitation from water if it 
became saturated with CaCQ3, and some limestone has been formed 
in this way. Rock-salt has been deposited in thick beds at various. 
times and places, as it is being deposited now about Salt Lake in 
Utah. The sodium of the salt (NaCl) doubtless came from decay- 
ing rock, for many igneous rocks contain a little sodium in some 
complex combination. In the decay of the rock, the sodium is 
taken out of its complex combination, and made into some soluble 
compound, and then taken to the sea or to a lake. Its union with 
chlorine makes common salt. Gypsum (CaSQu,) is another form of 


SEDIMENTARY ROCKS 269 


rock deposited in a similar way. Jron ore occurs in large bodies, 
and some of them were formed by precipitation. Salt, gypsum, 
limestone, and iron ore are peculiar among rocks in that but one 
mineral enters into their composition when they are pure. 

Coal is a sort of rock formed from accumulations of vegetable 
matter. Some other sedimentary rocks, as noted above, are formed 
organically, though they can hardly be said to be organic. 

The principal classes of sedimentary rocks are given below: 

( Conglomeratic rocks,— gravel, conglomerate, breccia, etc. 
Mechanically formed | Arenaceous rocks,— sand, sandstone, some arkose, etc. 


Clastic Argillaceous rocks,— clay, shale, etc. 
A few limestones. 


{ Some carbonate rocks, e. g., travertine, siderite. 
Chemically formed Chloride rocks,— especially rock-salt. 
Non-clastic Sulphate rocks,— especially gypsum. 
Some siliceous rocks,— some cherts, etc. 


Organically formed / Calcareous rocks,— most limestones. 
Non-clastic ~ Siliceous rocks,— siliceous oozes, sinter, etc. 
Carbonaceous,— coal, etc. 


Distinctive Features of Sedimentary Rocks 


Stratification. Most sedimentary rocks are arranged in more 
or less distinct layers; that is, they are stratified (Fig. 2). Stratifi- 
cation consists primarily in the superposition of layers one on an- 
other. Layers of like constitution or compactness may be sep- 
arated by films of different material which cause the partings. The 
bedded arrangement is due to various causes, but primarily to the 
varying agitation of the waters in which the sediments were laid 
down. Where depositing waters are agitated vigorously to the 
bottom, coarse sediment only is deposited. Where waters are 
quiet at the bottom, fine sediment is the rule. Since the agitation 
of waters is subject to frequent change, coarser sediment succeeds 
finer, and vice versa, in the same place. Hence arise beds, layers, 
and Jamine. ‘The terms layer and bed generally are used as syno- 
nyms, while /amine are thinner divisions of the same sort. The 
term stratum is sometimes applied to one layer, and sometimes to 
all the consecutive layers of the same sort of rock. For the latter 
meaning the term formation is often used. 

The commoner sorts of bedded rock are limestones, shales, 
sandstones, and conglomerates. In many places the bedding of 
limestone is caused by films of clayey matter between the layers, 


270 MATERIALS AND THEIR ARRANGEMENT 


the films causing natural partings. Bedding arises also from varia- 
tions in the physical condition of the calcareous sediment itself. 
Lamination is not, as a rule, conspicuous in pure limestone, though 
it may be in the shaly phases of this rock. Shales are normally 
laminated as well as bedded, and the lamination may be more 
notable than the thicker bedding. Bedding in shale may arise 
from the introduction of sandy lamine, or by changes in the texture 





Fig. 244. Cross-bedded sandstone. Maol Donn, Arran. The layers are 
horizontal, some of the laminz, diagonal. (H. M. Geol. Surv.) 


of the mud, etc., of which the shale was made. Some sandstones 
are divided into beds by shaly or clayey partings, or by variations 
in the coarseness or coherence of the sand itself. Sandstones may 
be thick or thin-bedded, and their bedding pe insensibly into 
lamination. 

Sand is deposited normally in relatively shallow’ water where 
it is subject to much shifting before it finds permanent lodgment. 


SEDIMENTARY ROCKS 271 


In the shifting, bars or reets are formed, most of which have a 
rather steep face in the direction in which the sand is shifted. The 
sand carried over the top of the bar finds lodgment on the sloping 
face beyond. The inclined laminz thus formed constitute a kind 
of bedding, but its planes do not conform to the general horizontal 
attitude of the formation as a whole. The structure is called 
cross-bedding, or, more accurately, cross-lamination (Fig. 244). 
The same structure is developed on delta fronts, and generally in 
water shallow enough to be subject to frequent agitation at the 
bottom. Sandstone is cross-bedded more commonly than shale 
or limestone. The bedding of conglomerate is due chiefly to varia- 
tions in coarseness. Lamine or beds of sand occur between the 
layers of coarser material in many places. The beds of conglomer- 
ate are likely to be thick, and in conglomerate cross-bedding is 
common. 

Lateral gradation. When the varying nature of the agitation 
of the sea at different depths and along the different parts of a 
coast is considered, it will be understood that deposits of one kind 
may grade into others horizontally. Thus a bed of conglomerate 
(gravel) may grade laterally into sandstone (sand), and this into 
shale (mud) or limestone. It is indeed more remarkable that sedi- 
mentary strata are as regular and persistent as they are, than that 
they grade into one another in some places. 

Position of strata. At the time of deposition, beds of sediment 
conform in a general way to the slope of the bottom where they are 
laid down. Since the slope of the sea bottom near shore is very 
gentle, as a rule, beds of sediment are, in most cases, nearly hori- 
zontal when deposited. Their slope is rarely so much as 20°, and 
commonly less than 5°. 

Special markings. The rhythmical action of waves gives rise 
to ripple-marks (Fig. 196), which are also made by streams, stream- 
like currents, and wind (Fig. 13). They are usually only a few 
inches from crest to crest, but in rare instances they attain much 
greater size. Under proper circumstances, ripple-marks are pre- 
served indefinitely. 

Some sediments are exposed between tides, or under other cir- 
cumstances, for periods long enough to permit drying and crack- 
ing at the surface. On the return of the waters, the cracks may be 
filled and permanently preserved. (Figs. 200 and 201) Such rec- 
ords of sun cracks affect shales chiefly, but are seen occasionally in 


202 MATERIALS AND THEIR ARRANGEMENT 


limestones and fine-grained sandstqnes. During the exposure of the 
sediments, a shower may pass, and raindrop impressions (Fig. 245) 
be made which are subsequently filed by fine sediment and preserved. 

Unconformities. Figs. 246, 247, and 248, show, in each case, one 
set of beds out of harmony with another set. This relation is one of 





Fig. 245. Raindrop impressions. (Brigham.) 





Fig. 2460. Unconformity between the Devonian (the thick-bedded rock) below, 
and the Coal Measures (thin-bedded and broken) above. Iowa. (Udden.) 


STRUCTURAL FEATURES 273 


unconformity. Most unconformities are developed by the erosion, 
the deformation, or both, of the older and lower set of beds, before 
the deposition of the younger and*upper set. The interval of time 





Fig. 247. Unconformity in Bighorn Basin, Wyoming. The lower (Laramie) 
beds dip notably to the left, and the upper horizontal (Wasatch) strata rest upon 
their cut-off edges. (Fisher, U. S. Geol. Surv.) 


between the deposition of the unconformable sets of beds may be 
very long — when the unconformity is great, or short — when the 
unconformity may be slight. Unconformities are of great signifi- 
cance in the interpretation of geological history. Unconformities 





Fig. 248. One phase of unconformity... The beds at the right were tilted to 
their present position before the deposition of the beds at the left. 


exist between stratified rock and igneous rock, and between strati- 
fied rock and metamorphic rock, as well as between different series 


of bedded rocks. 


Structural Features Arising from Disturbance 


Inclination and folding of strata. The original attitude of beds, 
whether formed by water or by lava-flows, commonly departs but 
little from horizontality. Locally, however, both kinds of deposits 
are made on considerable slopes. 


——~ 


274 MATERIALS AND THEIR ARRANGEMENT 





Fig. 249. Open anticlinal fold, near Hancock, Md. (U. S. Geol. Surv.) 





Fig. 250. Closed anticlinal fold, near Levis Station, Quebec. (U. S. Geol. 
Surv.) 


STRUCTURAL FEATURES 275 


Many sedimentary rocks and many lava flows have lost their 
original position through crustal movements, so that beds which were 
once horizontal now dip; that is, they depart from horizontality. 
The beds of a given region may all 
dip in one direction, or the dip may 
change from point to point. They 
may be folded, and the folds may be 
open (Fig. 249) or closed (Fig. 250). 
The beds of sedimentary rock may 
even be on edge (Fig. 251), having a ; 
dip of 90°. These diverse positions a piiaes Seen (Van Hise, 
in which strata are found are the 
result of disturbance subsequent to their deposition. 

Modifications of the original attitude result from earth move- 
ments, and the measurement of these modifications is an important 
part of field study. 
The position of beds 
is recorded in terms of 
dip and strike. The 
dip is the inclination 
of beds referred to a 
horizontal plane (Fig. 
252) and is usually 
measured by a clino- 
meter. In measuring 
the dip, the maximum angle of slope is always taken, and its direc- 
tion as well as its amount is recorded. Thus dip 4o°, S. 20° W., 
gives the full record of the position of the bed of rock under con- 
sideration. The strike | 
is the direction of the 
horizontal edge of dip- 
ping beds, or more gen- 
erally, the direction of a 
horizontal line along the 
outcropping edge of a anne 3 
dipping bed, as illus-_ - pee S 
trated in Fig. 252. Since Fig. 253. Recumbent anticline. (Van Hise, 
the strike is always at U- 8 Geol. Surv.) 
right angles to dip, strike need not be recorded if the direction of the 
dip is. Thus dip 40°, S. 20° W. is the same as dip 40°, strike N. 70° W. 








Fig. 252. Diagram illustrating dip and strike. 





276 MATERIALS AND THEIR ARRANGEMENT 


When beds incline in a single direction, they form a monocline. 
When they are arched up as in a fold, they form an anticline (Figs. 
249 and 250). ‘The anticline may depart from its simple form, as 





Fig. 254. Syncline, C. and O. canal, 3 miles west of Hancock, Md. Shale and 
sandstone, near base of the Silurian. (Walcott, U. S. Geol. Surv.) 


shown in Fig. 253. The downfold corresponding to an anticline is 
a syncline (Fig 254.) When beds assume the position shown in Fig. 
251, the folds are zsoclinal. When considerable tracts‘are bent so 


as to form great arches or great troughs with many minor undula- 


/ 4 
i, ‘ 
Ft * ; ; , A 
m 1 hiro. aN 
. - P, Ws GE GG MILES) 
I.E ie eee 
SAN | MY tA aes iY 
WRI WMD oe 
ANTE WRU SELIG 
SES Ney 
SSS) SEW WRG 
pay i 


Fig. 255. Generalized fan fold of the central massif of the Alps. (Heim.) 


STRUCTURAL FEATURES 277 
tions on the flanks of the larger, they are called geanticlines, or 
anticlinoria (Figs. 219 and 255), and geosynclines or synclinoria 
(Fig. 256). Folding 
may be accompan- 
ied by the develop- 
ment of slaty cleav- 
age (p. 293). 

As found in the 
field, most folds are 
much eroded, and in many cases completely truncated (Fig. 255). 
The structure is then determined by a careful record of dips and 


strikes. On the field 
ie e/ ee 





Fig. 256. Synclinori 
U.S. Geol. Surv.) 


5 


, Mt. Greylock, Mass. (Dale, 


map, the record 
may be made as 


Fig. 258 | shown in Figs. 257 

and 259, where the 

_ AN. free ends of the 

| -- lines with but one 

Fig. 257 Fig. 260 Fig. 259 ree end, Bore i 
Fig. 257. Map record of dip and strike, showing the direction of dip, 


synclinal structure. 

Fig. 258. Diagram showing the structure correspond- 
ing with Fig. 257, as seen in cross-section. 

Fig. 259. Map record of dip and strike showing 
anticlinal structure. 

Fig. 260. The structure of the area shown in Fig. 250, 
in cross-section. 


sents a syncline, and that in Fig. 259 an anticline. 


while the other lines 
represent the direc- 
tions of strike. Ap- 
plying this method, 
the structure shown 
in Fig. 257 repre- 
In cross-section, 


the structure represented by Fig. 257 is shown in Fig. 258; that 


of Fig. 250, in Fig. 260. 
Fig. 261 shows a doubly 
pitching anticline; that is, 
an anticline the axis of 
which dips down at either 
end. Fig. 262 shows a com- 
bination of synclines and 
anticlines, and Fig. 263 a 
cross-section along the line 
ab of Fig. 262. The out- 


PA 
Y 


A 


- Fig. 261. 


anticline. 


- 
F 
k 


r+tb$sett4setdT* 


Map record of dip and strike 
showing plunging (dipping down at ends) 


crops of rock where the dip and strike can be determined may be 
few and far between, but when they are sufficiently near one 


278 MATERIALS AND THEIR ARRANGEMENT 


another, the structure of the rock, as shown in Figs. 262 and 263, 
may be worked out, even though the surface is flat. 
Much the larger portion of the earth’s surface is occupied by 
beds that depart but little from their original horizontal attitude, 
Se but in many moun- 
tainous regions the 
beds have suffered 
bending, folding, 
crumpling, and crush- 
ing, in various degrees. 
Distortion is on the 
whole most consider- 
able in the most an- 
cient rocks. Distortion 
is assigned chiefly to 
lateral thrust arising 
. from the shrinkage of . 
Fig. 262. Map record of dip and strike showing the earth, as explained 
complex structure. : ? 
i b in Chapter VIII. 


Complicated  struc- 
I NSE’ / NN tures may be very dif- 
ficult of interpreta- 


tion. Thus overturned 
folds reverse the order 
of the strata in the under limb of the fold (Fig. 253). After such 
folds have been greatly eroded, so that their outer form is lost and 
their relations have become obscure, the beds are likely to be in- 
terpreted as though they lay in natu 
order. Thus Fig. 264 might ae a cS 
simple monoclinal structure, or any one of Fig. 264. Diagram rep- 
the complex structures tae in Figs. 265, resenting either isoclinal 
266, or 267, so far as dip and strike show. or movocinaljsteaceare 
Joints. The surface rocks of the earth are almost universally 
traversed by deep cracks called joints (Figs. 2 and 268). In most 
regions there are at least two systems of joints, the members of 
each system being roughly parallel, while those of the two systems, 
where there are two, are approximately at right angles. In regions 
of great disturbance, the number of sets of joints may be three, four, 
or even more. The joints of each set may be many yards apart, or 
in exceptional cases, inches, or even a fraction of one inch. 


€ 
a 
o 


+ 
rhe ith Pols Leta ae: On 


> 


nae 


Fig. 263. Cross-section of Fig. 262, along the line ab. 


STRUCTURAL FEATURES 279 


In horizontal rocks the joints approach verticality, but where 
the rocks have been deformed notably, the joint planes may have 


any position. In igneous and metamorphic rocks they may simu- 


2+ 
oe wer eae 
; ae my 
é o ‘ 
‘ et Pes. * 
=e na > ‘ * ., *. 
Yo Pe Cal : FY : * 
% wee 


i Meee NT ie : \ ; L \ \ 
*, - AS H ee SAY} $ : 5 ea \ . 
wey : ‘ay aye et, a ; : H f a \ \ = * H Se ‘ 5 
% oak 4 . Ex ae : 4 \ 4 ‘ H ‘ Sy a \ 
oe ates | \ con MEW RR 
‘ani Viale, ‘lal as a, 
‘ WO 3 * 2 ees iz. ; 9 ee j 
Fig. 265 Fig. 266 Fig. 267 
‘ Fig. 265. A possible interpretation of Fig. 264. (Dana.) 
Fig. 266. A possible interpretation of Fig! 264. (Dana.) 
Fig. 267. 


A possible interpretation of Fig. 264. (Dana.) 


late bedding planes (Fig. 269). Joints do not ordinarily show them- 
selves at the surface in regions where there is much mantle rock, 
but they are readily seen in the faces of cliffs, in quarries, and, in 
general, wherever rock is exposed. Though some of them extend 
to greater depths than rock has ever been penetrated, they are be- 
lieved to be relatively superficial phenomena. They must be limited 
to the zone of fracture, and most of them are probably much more 
narrowly limited. pes, joints end at the plane of contact of two 








Fig. 268. Jointed rocks, 


Cayuga Lake, N.Y. (Hall.) 


sorts of rock. Thus a joint extending down through limestone may 
end where shale is reached. Joints may be offset at the contact of 
layers or formations, and a single joint may give place to many 
smaller ones. All these phenomena may be explained by the vary- 
ing elasticity of various sorts of rock. Generally speaking, rigid 
rock is more readily jointed than that which is more yielding 


280 MATERIALS AND THEIR ARRANGEMENT 





Fig. 269. Tabular joints in granite. Summit of Goatfell, Arran. (H. M 
Geol. Surv.) : 





Fig. 270. A surface of sandstone marked by numerous joints, chiefly in tw 
rectangular sets. Near Kinghorn, Fife. (H.M. Geol. Surv.) : 


FAULTS 281 


Joints may remain closed, or may gape. They may be widened 
by solution, weathering, etc., and they may be filled by detritus 
from above, or by mineral matter deposited from solution (veins, 
p. 286). Many rich ore-veins are developed along joint-planes. 
(p. 45). 

Joints have been referred to various causes, among which 
tension, torsion, earthquakes, and shearing are the most important. 
Most of them may probably be referred to the tension or compres- 
sion connected with crustal movements.' In the formation of a 
simple fold, for example, tension-joints parallel with the fold will 
be developed if tension goes beyond the limit of elasticity of the 
rock involved. If the axis of a fold is not horizontal, that is, if it 
pitches, as it commonly does, a second set of tension-joints roughly 
perpendicular to the first may be developed. If the uplift is dome- 
shaped and sufficient to develop joints, they will radiate from the 
center. It is true that joints affect regions where the rocks have 
not been folded, and where they have been deformed but little, but 
deformation to a slight extent is well-nigh universal. Shrinkage is 
a cause of certain minor tension-jointing. The columnar structure 
of Javas and sun cracks are examples. These causes, however, are 
not believed to affect rock to great depths. 

Exceptionally, open joints are filled “y the intrusion of sedi- 
mentary material from beneath. Thus have arisen the remarkable 
sandstone dikes? of the West, especially of California. Some such 
dikes are several miles (nine at least) in length. The sand of these 
dikes was forced up from beneath a 
either by earthquake movements 
or by hydrostatic pressure. 

Faults. The beds on one 
side of a joint-plane or fissure 
may be raised or sunk relative 3 
to those on the opposite side. ~ : 
Such a displacement is one type pan a 
ofia fault (Figs. 271 and 272). Fig. 271. A normal fault. 
Fault-planes vary from verticality to horizontality. The angle by 

1 Van Hise. Principles of North American Pre-Cambrian Geology; 16th 
Ann. Rept., U. S. Geol. Surv., Pt. I, pp. 668-672. 

2 Diller. Bull. Geol. Soc. Am., Vol. I, pp. 441-442. Ibid., Hay, Vol. III, 
pp. 50-55; and Newsom, ibid., Vol. XIV, pp. 227-268. 

3 Various articles in Economic Geology, Vols. I and Il; Chamberlin, Fairchild, 
Jaggar, Ransome, Reid, Spurr, and Willis. 

















i. 
6 
5 
1 


282 MATERIALS AND THEIR ARRANGEMENT 


which the fault-plane departs from the vertical is the hade (bac, Fig. 
271). The vertical displacement (ac) is the throw, and the hori- 
zontal displacement (bc) the heave. The displacement is the amount 
of movement along the fault-plane, ab. The cliff above the edge 
of the downthrow side is a fault-scarp. In many cases the scarp 
has been destroyed by erosion; but a few fault-scarps of mountain- 
SO eee aon ous heights are known, as along 
Se a a some of the basin ranges of 
: Utah and Nevada. Most fault- 
peop mane reo? scarps which persist are much 
ae ES RIE: modified by erosion. 
4 Faults involving vertical dis- 
placement along joints are of 
‘two general classes, normal (or 
Fig. 272. A thrust-fault. The dotted gravity) and reversed (or thrust). 


lines at the left show the portion which ‘ 
has been removed by erosion. The pres- In the normal fault (Fig. 271) 


ent surface is shown by the line to the left the overhanging side is the 
of a. downthrow side, i. e., the down- 
throw is on the side towards which the fault-plane declines. Nor- 
mal faults, as a rule, indicate an extension of strata, this being 
necessary to permit one of the dissevered blocks to settle. In the 
thrust fault (Fig. 272), the overhanging beds appear to have moved 
up the slope of the fault-plane, as though the displacement took 





Fig. 273. Diagrams showing relations of faults and folds. 


place under lateral pressure. This is clearly shown to be the case 
where an overfold passes into a thrust fault. Another type of 
thrust-fault is shown in Fig. 273. 

In thrust-faults, the heave may be great. The eastern face of 
the Rocky Mountains near the boundary-line between the United 
States and Canada has been pushed over the strata of the bordering 


FAULTS 283 


plains to a distance of at least seven miles.1. Overthrusts of com- 
parable displacement have been detected in Scotland? and else- 
where. 

Some faults branch, and in some cases the faulting is along a 
series of parallel planes near one another, instead of being along a 
single plane. Such a fault is 
distributive (Fig. 274). Faults 
are found to die out when 
traced horizontally, in some 
cases by passing into mono- 
clinal folds (Fig. 275), and in 
some cases without connection 
with folding. In depth they 

robably die out in various Fig. 274. A branching fault. (Powell, 
ee (Fig. 276). A fault of eepeaa et) 
thousands, or even hundreds of feet is probably the sum of numer- 
ous slight slippings distributed through long intervals of time. The 
faulting along one plane may be the cause of many earthquakes. 





























Fig. 276. The fault above 
grades into a fold below. Thick- 
ening and thinning of layers next 
the fault-plane is evident. Based 

Fig. 275. Diagram of a fault pass- on experiments of Willis. (13th 
ing into a monoclinal fold. Ann. Rept., U. S. Geol. Surv.) 





The rock on either side of a fault-plane may be smoothed as 
the result of the friction of movement. Such smoothed surfaces are 
slickensides. 

The significance of gravity and thrust faults.’ Faults afford 
an indication of the conditions of stress and tension to which a 
region has been subjected, but some caution must be exercised in 


1 Willis, Bull. Geol. Soc. of Am., Vol. XIII, pp. 331-336, and McConneli, 
Canada Geol. and Nat. Hist. Surv., 1886, Pt. II. 

2 Geikie, Text-book of Geology. 

* Van Hise, Sixteenth Ann, Rept., U.S. Geol. Surv., Pt. I, pp. 672-678. 


284 MATERIALS AND THEIR ARRANGEMENT 


their interpretation. Most gravity faults indicate an extension 
of the surface sufficient to permit the fault-blocks to settle down 
unequally. Thrust faults, as a rule, signify a compression of the 
surface which required the blocks to overlap one another. In other 
words, normal faulting usually implies tensional stress, and reversed 
faulting compressional stress. Exceptional cases aside, the infer- 
ence from gravity faults is that the regions where they occur have 
undergone stretching, while the inference from thrust-faults is that 
the surface when they occur has undergone compression. 

In view of the current opinion that the crust of the earth has 
been subjected to great lateral thrust as a result of shrinking, it is 
well to make especial note of the fact that the faults which imply 
stretching are called normal because they are the more numerous; 
and that the faults which imply thrust are the less common. The 
testimony of normal faults in favor of tension is supported by the 
prevalence of gaping crevices, and veins. All these phenomena 
seem to testify to a stretched condition of the larger part of the 
surface of the continents. 

Faulting may bring about numerous complications in the out- 
crops of rock formations. These are difficult of detection in some 
cases, especially after erosion has destroyed the fault-scarps.1 

Faults of horizontal displacement. Horizontal displacement 
may take place along a joint-plane, with no vertical displacement. 
This also is faulting. Horizontal displacement accompanies verti- 
cal displacement, in many cases, and the former is as much a 
part of the faulting as the latter. The tendency of recent study, 
whether based on theory or on field observation, is to emphasize 
the importance of the horizontal movement in faulting. In many 
mines, for example, where the walls of shafts and tunnels afford 
excellent opportunity for observation, horizontal movement is more 
in evidence than vertical. 

There are various displacements of rock bodies not mentioned 
above which are akin to faulting, if not to be regarded as such. 
Thus when strata are folded there is some slipping of layer on layer. 
In places there is displacement of layer on layer, even when the beds 
are not folded. Such a case with a well developed ‘“‘slickenside”’ 
contact is known in Ohio, between beds which are nearly horizontal. 
The recognition of such movements as faults opens a wide door. 
The great variety of displacements along joints or other partings in 

1 See authors’ Geologic Processes, pp. 522-524. 


ALTERATIONS OF ROCKS 285 


the rock, shows the difficulty of defining faults sharply. Many 
movements of displacement, which can hardly be separated from 
faults logically, are not usually called faults. 


Map work. The sections of the Structure Section Sheets of the folios of the 
U. S. Geol. Surv. furnish abundant illustrations of a variety of structural features, 
such as folding and faulting, and the relations of sedimentary, metamorphic, and 
igneous rocks. The sections of various Bulletins, Professional Papers, etc., of the 
same Survey afford other illustrations. See also Exercise XVII in Interpretation of 
Topographic Maps. 


INTERNAL CHANGES IN IGNEOUS AND SEDIMENTARY ROCKS; 
METAMORPHISM 


We have seen already that igneous rocks undergo physical and 
chemical changes, whereby they are disintegrated, giving rise to 
what has been called rock waste: Similarly, sedimentary rocks may 
be decomposed and converted into waste. The waste from one 
generation of rock is the raw material for rock of a new generation. 
It is ‘‘rock waste’’ in somewhat the same sense that lumber is forest 
waste. 

Properly speaking, all changes which rocks undergo after being 
formed are metamorphic changes. According to this view, de- 
cayed rock is a phase of metamorphic rock; but it has been cus- 
tomary in the past to limit the term ‘‘metamorphic”’ to rocks which 
are made more compact, more complex in constitution, or more 
crystalline. Both sedimentary and igneous rocks may be meta- 
morphosed. 

Induration of sediments. The first step in the alteration of 
sediments is their induration, through the aid of cement, pressure, 
etc. Sandstone and shale are not commonly called metamorphic 
rocks, but they are metamorphosed sand and mud, respectively. 
The cementing material of sediment, as already noted, is mineral 
matter deposited from solution in water. Thus mineral matter 
dissolved at and near the surface may be carried down by descending 
water, and deposited between the grains of sediment, binding them 
together. The cementation may be slight, or it may go so far that 
all the spaces between the grains of sediment are filled. When the . 
spaces between sand grains are filled with silica, the rock becomes 
quartzite. Between loose sand at the one extreme, and quartzite 
at the other, there are all gradations. Quartzite is classed as meta- 
morphic rock, but it is formed by a continuation of the process which 
converts sand into sandstone. Important changes in rock are 


286 MATERIALS AND THEIR ARRANGEMENT 


brought about by the solution and re-deposition of mineral matter 
by the water in the rocks. This process may be called aqueous 
metamorphism, because of the important part played by water. 
Since water is present in almost all rocks down to considerable 
depths, the changes which it produces are nearly universal down 
to the depths to which it penetrates. 

Cavity filling. Cavities in rocks larger than the spaces between 
grains also receive deposits, if the waters entering them carry min- 





Fig. 277. Veins of calcite in volcanic tuff. Shore west of Kincraig Point, Elie, 
Fife. (H. M. Geol. Surv.) 
eral matter in solution. Thus joints or cracks may be filled with 
mineral matter, making veins (Fig. 277). The agates developed in 
some cavities afford another illustration of cavity filling. Here the 
successive layers are 
commonlyof quartz, 
differing from one 
another in color and 
texture. Geodes are 
cavities partly filled 
with crystals (Fig. 
278), mostly of 
quartz or calcite. 

Replacements. In 
both sedimentary 
and igneous rocks 
there are replace- 
ments. “Giie 
through the dissolv- 
ing and depositing 
Fig. 278. Geode. (Bassler, U. S. Geol. Surv.) action of water the 





ALTERATIONS OF ROCKS 287 


calcium carbonate of corals, shells, etc., may be replaced by silica. 
The substitution may take place so that even the minutest details 
of structure are preserved. Woody matter is, under proper condi- 
tions, replaced by silica, forming petrified wood. 

The material of one crystal may be replaced by different mate- 
rial, as the molecules of calcite by zinc carbonate. This gives a 
pseudomorph of zinc carbonate after calcite, the zinc carbonate 
taking the form of calcite, instead of the form which it would take 
if crystallizing under other circumstances. This sort of change may 
affect the crystals in any sort of rock. 

Concretions. Another phase of the internal reconstruction of 
sedimentary rocks is the assembling of matter of the same kind. 
For instance, silica that was deposited in the form of siliceous shells 





Fig. 279. Deposits of calcite (travertine, stalactites, and stalagmites) in 
Wyandotte Cave, Ind. (Hains.) 
and spicules of plants and animals, and disseminated through the 
sediments as they were deposited, may be aggregated later into 
nodules or concretions of chert or flint (Fig. 280). Similarly, con- 
cretions of calcium carbonate or iron carbonate grow in silts or 
muds. In many other cases, too, kind comes to kind. ; 


288 MATERIALS AND THEIR ARRANGEMENT 





Fig. 280. Nodule of chert, about half natural size. (Photo. by Church.) 


In general, concretions are made by the deposition of mineral 
matter which was in solution, about a nucleus. The nucleus 





Fig. 281. Irregular calcareous concretions. Ryegate, Vt. (Photo. by Church ) 


may be a leaf, a shell, or some bit of organic or inorganic matter. 
The material of the concretion probably comes, in most cases, 
from the immediately surrounding rock. Concretions are generally 


ALTERATIONS OF ROCKS . 289 


of matter unlike that of the rock in which they form. Thus con- 
cretions of calcium carbonate (Fig. 281) are common in clay, con- 
cretions of chert (silica) (Fig. 289) in limestone, and concretions of 
iron oxide in sandstone. 

Many concretions develop after the enclosing sediment was 
deposited. This is shown, in some cases, by the fact that bedding 
planes run through the concretions. Concretions also form in 
sediments during their deposition, and exceptionally, rock is made 
up chiefly of them. The chemical precipitates from the concen- 
trated waters of certain enclosed lakes may take the form of minute 
spherules. From a fancied re- 
semblance of these concretions 
to the roe of fish, the resulting 
rock was called odlite (Fig. 282). 
Odlite is now forming about 
some coral reefs, presumably 
from the precipitation of lime 
carbonate temporarily in solu- 
tion. Some considerable beds 
of limestone are odlitic. The 
calcium carbonate of such rock 
may be replaced by silica, 
leaving the odlitic structure in 
siliceous rock. Some beds of 
iron ore are concretionary. 
Thus there are widespread beds Te 52- Outi texture, About nat 
of “‘flax-seed” iron ore made 
up of concretions of iron oxide which, individually, resemble the 
seed which has given the ore its name. Some concretions develop 
cracks within themselves, and the cracks may be filled with mineral 
matter differing in composition or color from that of the original 
concretion (Fig. 283). Such concretions are called septaria. 

In size, concretions vary from microscopic dimensions to huge 
masses, 10 or more feet in diameter. The variations in shape 
are also great, conditions of growth having much to do with the 
form. A concretion which starts as a sphere may find growth 
easier in one plane than another, when it becomes discoid. ‘Two or 
more concretions may grow together, giving rise to complicated 
forms. 

None of the changes thus far mentioned (p. 285 e¢ seq) consti- 





290 MATERIALS AND THEIR ARRANGEMENT 


tute metamorphism, in the generally accepted meaning of the term, 
but all are metamorphic changes, if that term be given its largest 
meaning. 

Surface vs. deep-seated changes. Near the surface, the action 
of water commonly tends to the decomposition of rock; but below 
a few hundred, or at most a few thousand feet, its general effect is 





Fig. 283. Section of a septarian nodule (clay ironstone). About 34 natural 
size. (Geikie.) 
to solidify the rock, for at these depths deposition exceeds solu- 
tion, and oxidation, carbonation, etc., go on much more slowly than 
near the surface, or not at all. Oxidized and hydrated sediments 
may be buried to great depths, and under the pressure and perhaps 
the high temperature of these depths, deoxidation and dehydration 
may take place, with resulting diminution of volume. These 
changes at considerable depth are one phase of metamorphism, 
even according to the older use of the term. 

Incipient crystallization: A common metamorphic change in 
sedimentary rock is incipient crystallization. Some limestones and 
dolomites are made up largely of small crystals, though the mass 
was originally a calcareous mud or ooze. New crystals also are 


METAMORPHISM 291 


developed in shales and other sedimentary rocks out of materials 
already present, or with such additions as ground-water may make. 
Such changes take place even under ordinary conditions of heat and 
pressure, through the help of ground-water. 

_ Change in composition. Besides simple deposition in pores and 
cracks, mineral matter in solution may enter into combination with 
other mineral matter, giving rise to new and in some cases to more 
complex and more compact mineral substances. The changes 
effected in this way go on slowly, but in the long course of time, they 
may go so far that none of the original rock material remains in its 
original condition —all having entered into new combinations. 
Soapstone or steatite, serpentine, chloritic and talcose rocks, all of which 
occur in large bodies, were developed primarily through the chemi- 
cal rearrangement of the mineral matter of some older rock, with the 
addition of some mineral matter brought in by ground-water, and 
with the subtraction of some soluble matter. Their metamorphism 
is largely chemical. 

Other conditions favoring metamorphism. Besides water, heat 
and pressure favor the metamorphism of rocks. ‘Their action gives 
rise to three general cases, but these three blend indefinitely: (1) 
great heat without exceptional pressure, (2) exceptional pressure 
without great heat, and (3) great heat and great pressure acting to- 
gether. Exceptional heat arises especially from the intrusion of 
lavas, and from pressure. Exceptional pressure arises chiefly from 
the weight of overlying rocks, and from lateral thrust due to shrink- 
age of the earth. Thrust generates heat as well as_ pressure. 
The water in the rocks greatly facilitates the chemical and mineral- 
ogical changes favored by heat and pressure. 

Metamorphism by heat. When lava is poured out on the sur- 
face, it bakes the mantle-rock which it overflows. The extent of 
the baking depends on the mass and temperature of the lava. The 
nature of the effect is much the same as in the baking of brick. It 
consists in the dehydration of the material, its induration by welding 
due to partial fusion, and the development of new compounds. The 
time involved is short, the pressure slight, and the water action 
limited. If the heat were great enough, the loose material over 
which lava flows would be fused; but complete fusion does not usu- 
ally take place when lava spreads out on the surface. 

Intrusions of lava (p. 228) heat the surface above as well as that 
below. The heat of the lava can escape only through the neigh- 


292 MATERIALS AND THEIR ARRANGEMENT 


boring rock, and the changes effected by a given mass of lava are 
more considerable. Furthermore, the time during which the 
adjacent rock is hot, and therefore the time during which thermal 
waters are operative, is usually longer than in the case of extruded 
lavas, and the chemical and crystalline changes are greater. ‘The 
changes are greater the greater the mass of the lava and the higher 
its temperature. | 

In limestones and sandstones the changes are simple, and in 
shales more complex. In pure limestones and dolomites little chemi- 
cal change takes place, but the molecules are rearranged into larger 
crystals, making marble. The coarseness of the crystals is a rough 
sort of measure of the length of time during which the heat acts, 
and of its intensity; but much depends on the freedom of the attend- 
ant water circulation. If impurities, as silica, alumina, iron, etc., 
were present in the limestone, various silicate minerals may be 
formed in the marble. In pure quartzose sandstones, the effect is 
to bring about more complete cementation, converting the sandstone 
into quartzite (p. 285). Here, asin marbles, impurities form adventi- 
tious crystals. In shales, the material to be acted upon is more 
complex, for, while the main mass is composed of hydrous aluminum 
silicate, there is usually much free quartz, and in many cases some 
potash, soda, iron, compounds of calcium, magnesium, etc., for many 
muds from which shales arise contain not only the fully decom- 
posed matter of the original crystalline rocks, but some fine matter 
worn from them by wind and water without decomposition. When 
this mixed matter is acted upon by high heat and moisture, it tends 
to return to its original crystalline state, so far as its changed com- 
position permits. The result is the development of complex sili- 
cates, similar to those of igneous rocks, such as feldspar, mica, 
hornblende, etc. Mica schists are common products of the meta- 
morphism of shales by contact with bodies of lava. Mica schists 
also are formed in other ways, and other schists, dependent on the 
composition of the shales, are formed about intrusions of igneous 
rock. In all such cases pressure probably attends the heat, and is 
a factor in the development of the schists. When the change in- 
duced by the heat is less considerable, the shale is baked, with 
incipient re-crystallization, and may take the form of argillite, a 
compact, massive sort of shale. 

Beds of hydrous iron oxide (limonite) or of iron carbonate (sider- 
ite) may be converted by heat into hematite or magnetite (p. 255). 


METAMORPHISM 293 


Beds of peat, lignite, and bituminous coal are converted into anthra- 
cite by the driving off of the volatile hydrocarbons. If the process 
goes to the extreme, graphite is the result. 

Metamorphism by pressure. When rocks made up of clastic 
particles are compressed in one direction, and are relatively free to 





Fig. 284. Figure showing the elongation of pebbles under pressure. Carbon- 
iferous formation, Newport, R.I. (Walcott, U.S. Geol. Surv.) 


expand in others, the particles which are already elongated tend to 
turn so that their longer axes are at right angles to the direction of 
pressure, and all particles, whether elongate or not, are more or less 
flattened at right angles to the direction of stress. This is readily 
seen where the particles are large (Fig. 284). Asa result of the turn- 
ing (or orientation) and flattening of their particles, rocks so affected 
split more readily between the elongate and flattened particles than 
across them. In other words, the rocks cleave along planes normal 
to the direction of compression. ‘The structure thus induced is 
known as slaty structure (Fig. 285), and is illustrated by roofing- 
slate, which was originally a mud, later a shale, and finally assumed 
the slaty condition under strong compression. In some cases the 
original bedding may still be seen running across the cleavage planes 


204 MATERIALS AND THEIR ARRANGEMENT 





Fig. 285. Pre-Cambrian fossiliferous slate. Deep Creek Canyon, 16 miles 
southeast of Townsend, Mont. (Walcott, U. S. Geol. Surv.) 


developed by pressure (Fig. 286). As the original mud beds were 
horizontal or nearly so, and as thrust is most commonly horizontal 
or nearly so, the induced cleavage commonly crosses the bedding 
planes at a high angle. If the beds are tilted or bent before the 
development of the slaty cleavage, the angle between the bedding 
planes and the slaty cleavage may be small. 

Limestones, sandstones, and conglomerates are not so easily 
compressed as mudstones, and they commonly take on only an im- 
perfect cleavage normal to the direction of pressure. 

Foliation, schistosity. Extreme pressure in a given direction 
is capable of breaking down and deforming the most resistant rock. 
This must necessarily be attended with the evolution of heat, and 
thermal effects are combined with pressure effects. The first effect. 


METAMORPHISM 295 





Fig. 286. Slaty structure and its relation to bedding planes. Two miles south 
of Walland, Tenn. (Keith, U.S. Geol. Surv.) 


of the compression of such a rock as granite may be to crush it. It 
then becomes granular or fragmental, and is really a peculiar species 
of clastic rock (autoclastic). By further compression, the fragmented 
material may be pressed into layers or leaves, much as in the develop- 
ment of slaty cleavage; but as a result of the nature of the material, 
the cleavage is less perfect. These changes may be attended by 
more or less shearing of the material upon itself. The result is a 
foliated or schistose structure (Fig. 4), the most distinctive feature of 
highly metamorphic rock. A foliated structure may be developed 
even in the most massive rocks. ‘Thus granite may be transformed 
into gneiss — which is like a granite in composition, but has a 
foliated structure, and basalt may be converted into schist, a common 
term for foliated crystalline rocks. 

The kind of schist produced by metamorphism depends on the 
constitution of the rock metamorphosed. Basic rocks give rise to 
basic schists, and acidic rocks to acidic schists. It is obvious that 


296 MATERIALS AND THEIR ARRANGEMENT 


ordinary shales cannot become basic schists, because in the produc- 
tion of the muds from which shales are made, the bases were 
mostly removed; but shales which are highly calcareous and magne- 
sian may be changed basic schists (say hornblende schists) by meta- 
morphism. Schists are commonly named for the abundant cleav- 





Fig. 287. Porphyry rendered schistose by pressure. Near Green Park, Cald- 
well Co., N. C.. (Keith, U. S. Geol. Surv.) 


able mineral constituent, as mica schist (chiefly of quartz and mica), 
talc schist, chlorite schist, etc. 

The crystallizing processes of metamorphism are fundamentally 
similar to the processes by which rocks crystallize from lavas; but 
in metamorphism, the work is done chiefly by the aid of an agueous 
solution, while in the solidification of lavas the crystallization is 
from a mutual solution of the constituents in one another, where 
water was but an incident. 

Metamorphic rocks are of course subject to deformation and 
faulting, the same as sedimentary and igneous rocks. They are 
also subject to alteration through decay, or through the reorganiza- 
tion of their materials into new forms, 


METAMORPHISM 297 


VARIOUS CLASSIFICATIONS AND NOMENCLATURES 


From the foregoing sketch of the processes of rock-making it 
will be understood that the varieties of rocks are many, and that 
they may be defined, named, and classified on many different bases; 
for example: 

(1) If the mode of origin is chiefly in mind, rocks may be classed 
as igneous (lavas, tufis, etc.); metamorphic (schists, gneisses, an- 
thracite, etc.); and sedimentary. The last includes (a) aqueous 
(water-laid sediments, travertine, etc.); (6) eolian (wind-blown 
sand and dust); (c) glacial (deposits by glaciers); (d) organic (peat, 
coal, etc., and indirectly, limestone, infusorial earth, etc.). 

(2) On the basis of textural character, rocks are designated ve- 
sicular (pumice, scoria, etc.); glassy (obsidian); porphyritic (p. 248); 
granitic or phaneritic (pp. 248 and 262); compact, porous, earthy, 
arenaceous (sandy), schistose, etc. 

(3) If chemical composition is to be emphasized, they may be 
classed as siliceous, calcareous, carbonaceous, ferruginous, etc.; or, 
_in case of igneous rocks, as acidic, basic, or neutral. 

(4) If crystallinity is made the basis, igneous rocks are desig- 
nated phanerites (crystals distinct), aphanites (crystals very small), 
porphyries, glasses, etc. 

(5) On the basis of mineral composition, rocks are quartzose, 
micaceous, chloritic, pyritiferous, garnetiferous, etc. 

(6) Regarded as mineral aggregates, some rocks are simple and 
some complex. If simple, they are named from the dominant 
minerals, as dolomite, hornblendite, etc.; if complex, they take spe- 
cial names, as syenite, gabbro (pp. 260, 262), etc. 

(7) On the basis of structure of the mass, rocks are classed as 
massive, stratified, shaly, laminated, slaty, foliated, etc. 

(8) When physical state and genesis are considered, they are 
clastic, fragmental, or detrital, (conglomeratic, brecciated p. 263, 
arenaceous, argillaceous (clayey), etc.); or pyroclastic (tufaceous, 
agglomeratic, p. 263), etc. 

As one of these characteristics is most important in a given rock 
or in a given study, and another in another, no one classification is 
satisfactory in all cases. 


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HISTORICAL GEOLOGY 


CHAPTER XI 
THE ORIGIN OF THE EARTH 


The bedded rocks of the earth’s shell reveal its history far back 
into the past with great fidelity; but the record of the earlier ages 
is indistinct, and if we attempt to go back to the earth’s beginning, 
the indistinctness merges into obscurity. The rocks below the well- 
bedded strata are so broken and altered, and so cut up by intrusions, 
that their history is read with great difficulty. Still lower lies the 
inaccessible interior of the earth whose nature is more a matter of 
inference than knowledge. 

Some suggestions as to the origin of the earth are found in its 
relations to the other bodies of the solar system, and certain features 
of this system give pointed hints concerning its early history. The 
interpretation of these outside relations of the earth and of the secrets 
of its hidden interior is yet far from clear, and our only recourse is to 
hypotheses; but it is important that we study these hypotheses, and 
note the ways in which they enter into interpretations of the earth’s 
phenomena, for not a few of the leading doctrines of geology hang 
on some hypothesis of the earth’s beginning, and have no greater 
strength than the hypothesis on which they depend. 


HYPOTHESES 


It is the nearly unanimous conviction of astronomers that the 
solar system was evolved in some way from a nebula of some form. 
Until recently, astronomers so generally accepted the view of La- 
place that it came to be known as ‘*The Nebular Hypothesis’’; 
but the advance of knowledge makes it necessary to consider other 
hypotheses which postulate that the solar system arose from a 
nebula whose constitution and mode of evolution differed from that 


299 


300 ORIGIN OF THE EARTH 


assumed by Laplace. The leading hypotheses of the earth’s origin 
fall into three groups: 

1. The gaseous hypotheses, in which the parent nebula is assumed 
to have been formed of gas collected into a spheroid by gravity, and 
to have been evolved into the present solar system by loss of heat, 
and the separation of the outer parts into planets. The type of the 
class is the Laplacian hypothesis. 

2. The meteoritic hypotheses, in which the parent nebula is 
assumed to have been a swarm of meteorites, the members of which 
moved in diverse directions. Frequent collisions gave rise to heat, 
light, and vaporization. The swarm of meteorites is thought to 
have behaved essentially as a coarse gas, and the evolution of the 
system to have been dynamically like the preceding. 

3. The planetesimal hypothesis, in which the original constitu- 
ents of the nebula are assumed to have been small bodies, molecules 
or aggregates, moving in orbits about a common center and form- 
ing a disk-like system. ‘The evolution consisted in the gathering of 
these small bodies (planetesimals) into planets and satellites. Dy- 
namically, this hypothesis differs more from the other two than 
they do from each other. 

1. The Laplacian hypothesis. During the last century the 
Laplacian hypothesis was generally accepted, and geological theories 
as to the early states of the earth, and as to many later events in its 
history, were built upon it. The hypothesis is so well known that 
a few sentences will recall its essential features. It holds that the 
sun, the planets, and the satellites were once parts of a glowing, 
rotating, spheroidal, gaseous nebula, which was expanded enough 
to occupy the whole space of the solar system. The nebula was 
assumed to have cooled by radiation of heat, and in cooling to 
have shrunk. The shrinkage accelerated the rate of rotation, and 
this increased the equatorial bulge which rotation developed. The 
progressive increase of cooling, rotation, and bulging finally led 
to the separation of an equatorial ring. As this ring cooled and 
contracted, it was disrupted and its substance gathered into a 
planet whose orbit lay in the plane the ring had occupied. A series 
of rings, separated in this way, gave rise to the several planets in 
turn, while the central mass formed the sun. ‘The orbit of any planet 
bounds approximately the space assigned to the nebula at the birth 
of that planet. At the time of origin, the several planets were 
thought to be hot, gaseous, and rotating. Cooling and shrinkage 


NEBULAR HYPOTHESIS 301 


increased the rate of their rotation, and this caused equatorial 
bulging, till some of them, following the example of their parent 
body, shed rings which became satellites. 

In support of this theory many harmonies in the motions of the 
members of the solar system were cited, and in the early days of 
the hypothesis, existing nebulz were thought to give it support, 
for among them, as then known, there seemed to be nebulous aggre- 
gations in various stages of development, from diffuse nebulous 
masses to forms almost as concentrated as suns; but the best photo- 
graphs now taken fail to show that any follow the lines of this 
hypothesis. Grave difficulties arise from the dynamics of the 
theory, but without some knowledge of celestial mechanics, it is 
not possible to appreciate the full force of the arguments against 
it. Some of them may be stated briefly. 

1. In the evolution of a gaseous nebula, it is highly improbable 
that rings would be formed, for the molecules of gas would separate 
from the parent nebula one by one. 

2. Even if rings were formed, there are grave difficulties in 
their development into spheroids as set forth by this hypothesis. 

3. In the intensely hot condition of the assumed ring which 
was to form the earth and moon, its gravity could hardly have held 
its gases together. Even now the earth does not appear to hold 
permanently very light gases, though it holds the heavier ones. 

4. Itis probable that the material of a ring, such as the supposed 
earth-moon ring, would have cooled to solid particles long before 
it could collect into a spheroid. In this case no secondary ring to 
form a moon would be developed. | 

5. Theinner satellite of Mars (Phobos) revolves about that planet 
three times while the planet rotates once. According to theory, 
these motions must have corresponded at the time of separation, 
and since that time the planet should have increased its rotation 
by cooling. Its period of rotation should therefore be shorter than 
the period of the satellite’s revolution. Explanations have been 
suggested for this difficulty, but they do not meet the case. The 
small bodies that make up the inner edge of the inner ring of Saturn 
also revolve in about half the time that planet rotates. 

6. If the solar system were converted into a gaseous spheroid, 
with its matter distributed according to the laws of gases, and 
expanded to Neptune’s orbit, and if this nebula had the total 
momentum (technically, the moment of momentum) of the solar 


302 ORIGIN OF THE EARTH 


system, it would not have acquired a rate of rotation rapid enough 
to detach matter from its equator until it had contracted well within 
the orbit of the innermost planet. 

7. If the nebula were a spheroid of gas whose density followed 
the law of gases, and if it had a rotation rapid enough to shed rings 
from its equator as the theory supposes, its moment of momentum 
would need to have been very much greater than the system now 
possesses. This is at variance with the established law that the 
moment of momentum of such a system must remain constant. 
To separate Neptune, the moment of momentum would need to 
have been 200 times what it is; to separate Jupiter, 140 times; to 
separate the earth, 1,800 times. These are enormous discrepancies 
and they are not consistent with one another. 

8. Comparing the masses of the planets with the moments 
of momenta they carried off from the parent nebula, strange dis- 
crepancies appear. The matter in the ring supposed to have formed 
Jupiter and his moons had a mass less than '/1ooo that of the 
nebula at the time of separation; but Jupiter and his moons have 
about g5 per cent of the total moment of momentum which the 
nebula then had. ‘The Laplacian hypothesis asks us 'to believe that 
an equatorial ring, having a mass less than 1/1000 that of the parent 
body, carried off 95 per cent of the total moment of momentum 
when it separated. The supposed separation of other rings involves 
similar incredible ratios. 

g. Under the Laplacian hypothesis, the satellites should all 
revolve about their planets in the direction in which the planets 
rotate on their axes; but the sixth satellite of Jupiter and the ninth 
satellite of Saturn revolve in the opposite direction. 

to. Our knowledge of nebule has been extended greatly in 
recent years, but nebule with such rings as the Laplacian hypothe- 
sis calls for have not been found. 

The force of these objections appears to be such as to make the 
hypothesis untenable. 

2. The meteoritic hypotheses. It was long ago noted that 
shooting stars enter the upper atmosphere in great numbers, and 
that occasional fragments of stony and metallic matter fall to the 
earth. Out of this grew the notion that the earth may have been 
built up in this way, save that the process was more rapid in the 
early days of the earth’s history. This notion, however simple 
and natural, may be dismissed without serious consideration, for 


METEORITIC HYPOTHESES 303 
the different directions of motion and the various velocities of 
meteorites are such as to forbid the belief that the solar system, with 
its symmetrical discoidal form and its harmonious motions, could 
have been formed in this way. 

A more logical meteoritic hypothesis is based on the conception 
that meteorites may be aggregated into swarms and constitute 
nebule. This hypothesis _ is, 
therefore, nebulo-meteoritic. 
Sir George Darwin came to the 
conclusion that such a swarm 
of meteorites would act very 
much like a gas, and that the 
laws of gases could be applied 
in determining its mechanics. 
If the meteorites of such a 
nebula move in various direc- 
tions, this hypothesis, as ap- 
plied to the origin of the earth, 
is practically identical with the 
gaseous hypothesis; and as 





applied to the solar system, it 
is subject to the criticisms al- 


ready urged against that hy- © 


pothesis. The term meteoritic 
hypothesis is used commonly 
in the above sense. It was ap- 
plied by its authors (Lockyer 
and Darwin) chiefly to the 
earlier and more scattered con- 
ditions of the nebule, and has 
not been applied specifically to 
the formation of a planet. If, 


Fig. 288. A spiral nebula in Canes 


Venatici, Messier 51. The exposure was 
long and has given relative exaggeration 
to the fainter parts. The nucleus is 
apparently dense and relatively massive; 
the coiling is pronounced and rather sym- 
metrical in the inner parts, but departs 
from symmetry in the outer parts. A 
notable feature is the comet-like streamers 
of some of the knots and denser portions. 
If these are true streamers, curved by 
motion, they imply an active rotation, 
and strengthen the inference drawn from 
the coiled condition. (Photo. by Ritchey, 
Yerkes Observatory.) 


on the other hand, the meteorites were so assembled as to have con- 
centric orbits and form a disk-like system, the system, to all intents 
and purposes, falls into class 3. 

3. The planetesimal hypothesis. When the shortcomings of 
the Laplacian hypothesis were seen to be so serious that there was 
no apparent way of escape from them, an alternative better in accord 
with the facts was sought. 

It has been shown by photography that there are a multitude of 


304. ORIGIN OF THE EARTH 


nebule,— at least ten times as many as were known a few years 
ago,— and that in this multitude there is one dominant form, the 
spiral nebula (Fig. 288). The spiral nebula has a central nucleus, 
from which two arms or sets of arms project on opposite sides, and 
curve spirally outward. The arms of some nebule branch, and are 
much interrupted and knotted, and between them there is much 
scattered hazy matter. The prevalence of this form of nebula 
implies that it is due to some process which has been common. The 
numerous nebulous knots on the arms, and in some cases more or less 
outside them, are significant features. Clearly the matter of the 
nebula is very unequally distributed, and does not conform to the 
laws of gaseous distribution. 

Recent advances in spectroscopy throw much light on the con- 
stitution of nebula. As inferred from their forms, the spiral 
nebula seem to be composed 
of solid or liquid particles, 
though gases may be present 
particularly in their nuclei and 
knots. These tiny bodies are 
believed to revolve about the 
center of gravity of the nebula, 
like little planets (planetesi- 
mals), but this has not yet been 
proved. ‘The planetesimal hy- 
pothesis is based on a spiral 
nebula of this supposed organi- 
zation.! 

The planetesimal hypothe- 
sis starts with a spiral nebula 
Fig. 289. A typical spiral nebula in consisting of the following ele- 


Piscium, Messier 74, with very symmetri- : 
cal arms, pronounced nucleus and knots, ments: (1) The main knots to 


and a relatively limited amount of nebu- serve as nuclei for the planets, 
lous haze. (Photo. from Lick Observa- (2) small scattered knots as the 
se nuclei of asteroids, (3) other 
small knots near to the large ones and controlled by them, as the 
nuclei of satellites, and (4) scattered matter or nebulous haze to 
be gathered into these nuclei to give them their mature sizes, and 
(5) the great central mass of the nebula, forming the nucleus of 





1 The manner in which it may have arisen is discussed in the authors’ larger 
work on Geology, Vol. II. 


PLANETESIMAL HYPOTHESIS 3053 


the sun. The gathering of the scattered planetesimals into the 
knots to form the planets, planetoids, and satellites is assigned to 
the coming together of these bodies as they pursued their slightly 
different orbits, not as the result of falling directly together under 
the control of gravity. 

It is assumed that the planetesimals had rather highly elliptical 
orbits arranged in disk-like form. Such orbits would be favorable for 
the meeting and union of the bodies following them. It can be shown 
mathematically that under such conditions the addition of planet- 
esimals to the nuclei would give them more and more circular orbits 
as the nuclei grew, and it is significant that most of the planetoids 
(asteroids), which presumably have grown little, have the most 
eccentric orbits, that Mercury and Mars, the smallest of the planets, 
have more eccentric orbits than the others, while the orbits of the 
larger planets approach circularity more closely. ‘The photographs 
of spiral nebulz show large knots with small ones near them, which 
appear quite capable of evolution into planets and satellites. They 
also show small scattered knots susceptible of forming planetoids 
(asteroids). The earth-moon system is assumed to have been 
derived from companion nuclei of very unequal sizes. 

The knots might have had a rotary motion at the outset, arising 
from inequalities of projection at the time of their formation; but in 
part, the rotations of the planets are assigned to the impacts of the 
planetesimals as they joined the nuclei to form the planets. There 
would be no fixed relation between the time of rotation of a planet 
and. the time of revolution of its satellites; the period of the latter 
might be longer or shorter than that of theformer. Evenif the revolu- 
tion-period of a satellite-nucleus was originally the same as the rota- 
tion-period of the planetary-nucleus, the growth of the planet might 
draw the satellite nearer to itself and shorten the time of its revo- 
jution. Thus the difficulty of Phobos and of the innermost part of 
the ring of Saturn is obviated.—The mode of accretion assigned 
- might give rise to forward rotation or to retrograde rotation of the 
planets and satellites; the forward rotation should be the rule and 
retrograde rotation the exception, as is the case. In a spiral nebula 
formed in the way assigned, the outer parts of the arms should be 
composed of lighter materials than the inner parts, and since the 
planets were formed from these arms, the inner ones should have 
higher specific gravities than the outer ones, as is the fact. Other 
peculiarities of the solar system seem to find a fitting explanation 


306 ORIGIN OF THE EARTH 





Fig. 290. Theoretical restoration of the parent nebula of the solar system. 
The nuclei of the several planets may be identified by their distances from the 
center. The dimensions of the inner parts are made disproportionately large. 


in the planetesimal hypothesis, but most of these must be passed 
without mention here. 

The assumed meetings and unions of planetesimals and nuclei at 
the crossings of their orbits imply a relatively slow evolution of the 
nebula into the solar system. The planetesimal hypothesis therefore 
implies a slow growth of the earth. With such a mode of growth, 
the stages of the earth’s early history depart widely from those 
postulated by the Laplacian and the meteoritic hypotheses. 


‘CHAPTER XII 


STAGES OF THE EARTH’S HISTORY PRIOR TO 
THE KNOWN ERAS 


The conception of the history of the earth prior to the earliest 
stage which can be read from the strata must depend upon the view 
which is entertained as to its origin. The course of its early history 
according to each hypothesis of its origin, will be sketched sepa- 
rately. Though these sketches are necessarily hypothetical, their 
study is important, for the great features of the earth and of the 
earth-shaping processes were inherited from these early stages. 


I. STAGES UNDER THE LAPLACIAN HYPOTHESIS 


The hypothetical stages of the earth’s early history, according 
to the Laplacian view have been stated as follows,! and they must 
have been essentially the same for any view of primitive conditions 
that involves a molten globe. 


I. ‘The Astral zon, or that of the fluid globe having a heavy vaporous envelope 
containing the future water of the globe or its dissociated elements, and other 
heavy vapors or gases. 

II. The Azoic eon. Without life. 

1. The Lithic Era: Commencing with the earth a solid globe, or at least 
solid at the surface; the temperature at the beginning above 2,500° F.; the 
atmosphere still containing all the water of the globe (estimated at 200 atmos- 
pheres), all the carbonic acid now in limestone and that corresponding to the 
carbon now in carbonaceous and organic substances (probably 50 atmos- 
pheres), all the oxygen since shut up in the rocks by oxidation, as well as that 
of the atmosphere and of organic tissues. ‘The time when lateral pressure for 
crustal disturbance and orographic work was begun; when “‘statical meta- 
morphism,” or that dependent on heat of a statical source — the earth’s 
mass and the vapors about it,— began. 

2. The Oceanic Era: Commencing with the waters condensed into an ocean 
over the earth, or in an oceanic depression, with finally some emerging lands, 
the temperature perhaps about 500° F., if the atmospheric pressure was still 
50 atmospheres. ‘The first of tides and the beginning of the retardation of 
the earth’s rotation. Oceanic waves and currents and embryo rivers begin 
work about the emerged and emerging lands; the large excess of carbonic 
acid and oxygen in the air and water a source of rock-destruction; before the 


1 Dana, Manual of Geology. 
307 


308 EARLY STAGES OF EARTH’S HISTORY 


close of the era, the formation of limestones and iron-carbonate by chemical 
methods, removing carbonic acid from the air and so commencing its purifi- 
cation; the accumulation of sediments without immediate crystallization or 
metamorphism, and thereby the beginning of the earth’s supercrust. 

III. The Archeozoic won. Life in its lowest forms in existence. 

1. The Era of the First Plants: Algz, and later of aquatic Fungi (Bacteria), 
commencing with the mean temperature of the ocean at possibiy 150° F., 
since plants mow live in waters up to and even above 180° F. Limestones 
formed from vegetable secretions, and silica deposits from silica secretions; 
iron-carbonate, and perhaps iron-oxides formed through the aid of the carbonic 
acid of the atmosphere and water; large sedimentary accumulation, where 
conditions favored, thickening the supercrust. 

2. The Era of the First Animal Life: Mean temperature at the beginning 
probably about 115° F., and at the end go° F., or lower; limestones and silica 
deposits formed from animal secretions; deposits of iron-carbonate and iron- 
oxides continued; large sedimentary accumulations.” 


Difficulties 


Quite apart from the objections to the Laplacian hypothesis, 
stated in the last chapter, two serious questions exist relative to the 
stages sketched above. The one grows out of the failure to find 
any great formation beneath all others having the distinctive char- 
acteristics of an original crust; and the other from doubt as to the 
possibility of the prodigious atmosphere postulated. 

Relative to an original crust. The theory of a molten earth 
carries the presumption that the liquid substance of the earth was 
arranged so that the heaviest matter was at the center and the 
lightest on the outside. As the granitoids are the lightest of the 
large classes of igneous rocks, the granite-like magmas should have 
formed the outer zone of the molten earth. The solid crust should | 
have been light (for rock) and homogeneous, and it should have 
formed a stratum over the whole earth. Except at the very surface, 
it should have been completely crystallized, for the cooling must 
have been very slow, a condition favorable for the growth of crystals. 
No very large amount of fragmental volcanic material can be as- 
sumed to have covered the original crust if the atmosphere contained 
all the water of the future hydrosphere, for that would allow no 
steam in the molten globe to produce abundant volcanic fragments. 
Pyroclastic material of later times can hardly be supposed to have 
concealed the original crust permanently, for many thousands of 
feet of rock have been eroded from the surface of the oldest known 
areas. It is equally improbable that the original crust has been 
concealed everywhere beneath sediments derived from itself. 


UNDER LAPLACIAN HYPOTHESIS 309 


Until recently, the great granitoid areas of the Archean system 
(the oldest known rocks) were thought to possess these obvious 
characteristics of an original crust; but it has been found that most 
of them were zntruded into rocks which had previously been formed 
on an older surface by (1) lava outflows, (2) volcanic explosions, and 
(3) sedimentation. This reduces to an unknown, and apparently 
to a vanishing quantity the rocks that can be referred plausibly 
to a supposed original crust. If further investigation shall finally 
exclude all accessible rocks from an original crust, the molten theory 
will have lost its observational support. 

Relative to the primitive atmosphere. Under the Laplacian 
hypothesis, the primitive atmosphere has been held to have been 
vast, hot, and heavy, and to have contained (1) all the water of the 
globe, (2) all the carbon dioxide now in carbonated rocks, (3) that 
portion of the oxygen which has been added to the rocks by oxida- 
tion, as well as (4) that portion of all these constituents which is 
now found in the atmosphere and in organic tissues. ‘The assump- 
tion back of this seems to be that heat always promotes the expulsion 
of gases from rock; if so, the exclusion of the gases from the rock should 
have been most complete in the white-hot primitive globe. The 
conception that the rocks after cooling re-absorb the atmospheric 
gases is expressed in the view, once prevalent, that the former 
atmosphere and hydrosphere of the moon have been absorbed into 
it, and in the familiar prophecies of a similar doom for the atmo- 
sphere and hydrosphere of the earth. 

Adverse evidence. So great an atmosphere with so much carbon 
dioxide and water-vapor should have given the earth a warm and 
equable climate. Such climates indeed seem to have prevailed at 
certain times during the earlier parts of the earth’s history, as 
during the later; but the studies of the past two decades have shown 
that there was extensive glaciation on the very borders of the tropics, 
as early as the close of the Paleozoic, and that there was glaciation in 
northwestern Europe, in China in Lat. 31°, in Australia, and perhaps 
in South Africa, at the very beginning of the Paleozoic. It is even 
claimed that there was glaciation in the early part of the Proterozoic 
long before the Paleozoic, and this claim seems likely to be made 
good. ‘There seem to have been, even in very early times, much the 
same alternations of uniform with diversified climates that have 
marked later eras. The air-breathing animals of early ages, and the 
devices that protected the leaves of plants against too intense sun- 


310 EARLY STAGES OF EARTH’S HISTORY 


light and too rapid evaporation, seem irreconcilable with a vast 
cloudy atmosphere overcharged with carbon dioxide and water- 
vapor. The hypothesis of an enormous original atmosphere, suffer- 
ing gradual depletion, finds, therefore, scant support in a critical 
study of either the biological or the physical history of the earth. 

Modifications of the Laplacian hypothesis (known commonly 
as the Nebular hypothesis) have been suggested,’ with a view to 
obviating the objections to the current form of the hypothesis as 
applied to the earth. But the suggested changes do not seem very 
satisfactory, and there is reason for thinking that all hypotheses 
of the earth’s origin involving a molten condition of the globe, will 
soon be abandoned by geologists. 


II. STAGES OF GROWTH UNDER THE PLANETESIMAL HYPOTHESIS 


It is possible to suppose that the earth grew up by accessions 
in some other mode than that of planetesimal evolution, but the 
latter furnishes the basis for the following sketch: 

1. Nuclear stage. A knot of the nebula was the nucleus of 
earth growth. The knot may have been gaseous, or planetesimal, 
or both. It caught planetesimals from the nebular haze as it 
crossed their paths, and thus grew in mass while it was being con- 
densed into the beginning of the earth-body. ‘This stage lasted 
until the knot was condensed into a solid mass. This mass then 
served as the nucleus for further growth by captured plane- 
tesimals. 

2. Initialatmospheric stage. There may possibly have been an 
early stage when the mass of the earth was too small to hold perma- 
nently the lighter free molecules such as form our present atmos- 
phere. In this case the nucleus must have been made up mainly of 
heavy molecules such as form the stony and metallic parts of the 
earth; but such a stage is not probable. If the mass of the nucleus 
were one-tenth of that of the present earth, it would hold some 
atmosphere, but it would be thin and composed mainly of the 
heavier gases. This early thin atmosphere grew as the earth grew, 
by capturing molecules from the nebulous mass. The stony and 
metallic planetesimals also contained atmospheric material in 
combination or occluded,? and some of this, set free when the 
planetesimals were heated by plunging into the air or when they 


1 Vol. II of the author’s three-volume Geology. 
2 The Gases in Rocks, R. T. Chamberlin, Carnegie Institution, 1908. 


UNDER PLANETESIMAL HYPOTHESIS SDL 


struck the earth, added to the atmosphere. Thus the atmosphere 
grew as the earth-body itself grew. Volcanoes, when they came 
into action, also added to the atmosphere, for they discharge much 
gas. ‘This picture of the early atmosphere is very different from the 
vast hot vaporous atmosphere of the supposed molten earth. 

3. Initial volcanic stage. As the earth grew and its gravity 
increased, its interior became more and more compressed and 
therefore more and more heated. Radio-active matter was no 
doubt gathered in with the other matter, and this developed heat. 
When the heat from these two sources became sufficient to liquefy 
the most fusible portions of the earth matter in particular spots, 
the fluid parts began to work their way toward the surface by fluxing. 
Other fusible matter was picked up on the way, and the radio-active 
matter in particular joined the rising threads of lava. When this 
rising lava reached the surface, volcanic action was inaugurated. 
According to this view, volcanoes do not originate from a ‘‘ molten 
interior,” or from ‘‘reservoirs” of molten matter left over from a 
general molten state, but the lava is generated from time to time 
by the continued action of radio-active substances, conjoined with 
the effects of compression and molecular rearrangement within the 
earth. ‘The heat of the interior of the earth is thus carried outward 
about as fast as it liquefies the more fusible parts within its reach. 
Thus the interior of the earth only reaches the temperature neces- 
sary to melt the more fusible parts, leaving the earth as a whole solid 
all the time. 

4. Initial hydrospheric stage. Water in the form of vapor is 
light and active, and probably was not the first gas to be held by 
the growing earth; but when the earth became large enough, water- 
vapor was held in the atmosphere, and when at length saturation 
was reached, it condensed into water and initiated the hydrosphere. 
The source of water, according to the hypothesis, was the same 
as that of atmospheric gases. 

It may be added that the hypothesis gives a simple explanation 
of ocean basins and continental protuberances. Because of unequal 
growth, the surface of the earth may never have been perfectly 
spheroidal, so that when water began to accumulate on its surface, 
it gathered in depressions. The planetesimal material which after- 
wards fell into the water was protected from weathering, while that 
which fell on the higher land was exposed to weathering, with its 
attendant lessening of specific gravity. ‘Thus the depressed areas 


312 EARLY STAGES OF EARTH’S HISTORY 


tended toward higher specific gravities, and hence toward still 
further depression when deforming stresses were brought to bear 
on them, while the elevated areas tended to grow relatively lighter, 
and to suffer relative elevation, under the stress of deformative 
movements. ‘Thus the differentiation of the oceanic basins from 
the continental masses began as soon as the hydrosphere began, that 
is, long before the earth reached its present size, and has continued 
to the present time. } 

5. Initial life stage. Suitable conditions for life seem to have 
existed after an atmosphere and hydrosphere had developed to the 
proper extent, but it seems possible that life began long before the 
earth was full-grown. Under the planetesimal hypothesis, therefore, 
the time during which life may have existed on the earth is very 
much longer than the time assumed under the older hypotheses. 

6. Climax of volcanic action. While volcanic action may have 
begun early, it probably had to await (1) sufficient growth to give 
the requisite internal heat by compression, and (2) sufficient time 
for the heat so developed to creep out to zones of less pressure, 
where it would suffice to liquefy the more fusible (soluble) parts of 
the rock. Vulcanism was probably hastened by radio-activity. 
Once begun, it is believed to have increased in importance, reaching 
its climax some time after the more rapid growth of the earth had 
ceased. 

For obvious reasons, the climax of vulcanism was attended by 
deformations of exceptional intensity. The transfer of so much 
material from below to the surface required readjustment within, 
and the intrusion of the enormous granitic batholiths, such as are 
found in the early formations, was in itself a cause of deformation. 
Diastrophism probably had its climax with the climax of vulcanism, 
and both came, by hypothesis, about the time of the opening chapter 
of the well-recorded history of the earth. The formations of the 
period when volcanic action was at its height, including some con- 
temporaneous sedimentary deposits, are regarded as constituting 
the oldest accessible rocks of the earth (the Archean), though prob- 
ably only the later part of the great volcanic series is represented by 
the known Archean. It is for each student to judge whether the 
assigned antecedents lead felicitously or otherwise to the condi- 
tions which the oldest known rocks reveal. The value of a hypothe- 
sis, when its truth cannot be demonstrated, lies mainly in its work- 
ing qualities. 


UNDER PLANETESIMAL HYPOTHESIS 313 


7. Gradational stage. To complete the survey of stages, it 
is necessary to note that after the growth of the earth had nearly 
ceased, and volcanic action had passed its climax, the surface was 
no longer subject to universal burial, but was exposed, age after age, 
to the action of air and water. The material removed by these 
agents from the higher parts was deposited in the basins. Through- 
out all the remaining part of this stage, the dominant geologic 
processes were gradational. Vulcanism and diastrophism continued 
to be important, but not dominant. This stage embraces the Pro- 
terozoic and later eras. 

These stages of the earth’s history may be grouped as follows: 


III. Eon of Dominant Gradational | Cenozoic Era 


Processes (the well known Mesozoic Era 
eras) Paleozoic Era 


Proterozoic Era 


II. Eon of Dominant Extrusive 
Processes (transitional from 
the hypothetical to earliest 
known era) 


Archeozoic Era 
b) The known portion 
i a) The buried portion 


Initial life stage 
; Initial hydrospheric stage 
I. Eon of Dominant Formational | Initial volcanic stage 
Processes (hypothetical) Initial atmospheric stage 
Gee stage (Early nuclear growth) 


Nebular stage 


CHAPTER XIII 
THE ARCHEOZOIC ERA 


Though the preceding sketches of the early stages of the earth’s 
history are but hypothetical, they afford a helpful introduction to 
the study of that part of the earth’s history recorded in the rocks. 
Figs. 291-293 represent diagrammatic radial sections which illus- 
trate the different conceptions of the constitution of the earth. 
The following summary should make the figures clear: 

1. According to the Laplacian hypothesis, there should be pre- 
sedimentary igneous or meta-igneous rock everywhere below the 
prevailing sedimentary rocks of the surface. The plane of demark- 
ation between these two sorts of rock should, as a rule, be distinct. 

A modification of the Laplacian hypothesis, so far as applied to earth, pos- 
tulates that much gas and vapor were occluded in the molten earth, instead of being 
all in the atmosphere. On this assumption, it is conceived that there might have 
been a period of great vulcanism after the formation of a crust, and that the 
original crust was covered deeply with extruded rock (Fig. 292). If this were the 
case, the original crust might not be accessible. On the meteoritic hypothesis of 
the earth’s. origin, the conditions would have been much as on the planetismal 
hypothesis so far as concerns the oldest rocks accessible. 


2. According to the planetesimal theory, (1) the core of the 
earth (Fig. 293) is made up of planetesimal matter. After aggrega- 
tion, this matter was probably re-crystallized under the influence 
of the heat and pressure which the aggregation involved, the result- 
ing rock being essentially igneous in its nature. Outside the central 
core there should be (2) a thick zone made up largely of planet- 
esimal matter, but partly of igneous rock erupted from below, and 
partly of sedimentary rocks formed at the surface at all stages of 
the earth’s growth after the hydrosphere came into existence. 
The planetesimal matter is assumed to predominate in the lower and 
major part of this zone. Igneous rock is assumed to have a some- 
what irregular distribution through it, while sedimentary rock 
increases in importance above, but remains throughout a subordinate 
constituent. This zone records the growth of the earth from the 
beginning of volcanic and atmospheric processes, until it reached 


314 


COMPOSITION OF EARTH 315 





LR AR Re 





Primitive igneous 


Primitive igneous 
rock. k 


rock. 
















Planetesimal matter 
predominant; igneous 
rock abundant; sedi- 
mentary rock a minor 
constituent. increasing 
toward the surface. 








Planetesimal] 
matter with 
more or less 
igneous 
rock. 


Fig 293 


Fig. 291. A diagrammatic sector of the earth illustrating its structure accord- 
ing to the Laplacian hypothesis. The great body of the earth is made up of the 
original igneous rock. Sedimentary rocks, together with some extrusive rocks, 
make but a thin coating, represented in the diagram by black, outside the great 
igneous interior. The original igneous rock is represented as appearing at the sur- 
face insome places (AR). This, according to one view, might represent the Archean 
rock. 

Fig. 292. Diagram illustrating the composition of the earth on the modified 
form of the Laplacian hypothesis. The great body of the earth is the original 
igneous rock. Outside this original igneous mass, there is a zone (zone 2) of ex- 
trusive material, with perhaps some sedimentary rock interbedded. The material 
of this zone is represented as coming to the surface at some points (A). Outside 
this zone there is a third, made up primarily of sedimentary, but subordinately, of 
extrusive rocks. The material of the second zone might constitute the Archean 
rock. 

Fig. 293. Diagram representing the structure of the earth according to the 
planetesimal hypothesis. The material of zones 1 and 2 is indicated in the diagram. 
Zone 3 of this figure corresponds to zone 2 of Fig. 292, and zone 4 of this figure corre- 
sponds to the outermost zone of Figs. 291 and 292. 


316 ARCHEOZOIC ERA 


nearly its present size. The central core and this thick.zone about 
it represent the Formative Eon (p. 313). (3) The next zone, rela- 
tively thin, is assumed to be made up largely of extrusive igineous 
rocks, with subordinate amounts of sediment, and matter gathered 
from space. This zone represents the Extrusive Eon (p. 313). 
(4) On the outside lies the superficial zone in which sedimentary 
rocks predominate, though associated with not a little rock of igne- 
ous origin. This fails to cover the globe completely. 

The oldest rocks. The deepest excavations yet made in the 
earth are little more than a mile deep. Because of deformation and 
erosion, rocks once at much greater depths are now exposed; but 
the maximum thickness of rocks open to observation is no more than 
afew miles. Definite knowledge of rock formations and structures 
is therefore limited to some such thickness. (1) According to the 
gaseo-molten hypothesis, we might hope to reach the originai crust; 
for it is not to be supposed that this original crust is everywhere 
covered so deeply by material derived from it as to be inaccessible. 
(2) According to the modified form of this hypothesis (Fig. 292), the 
oldest accessible rock should be in the zone of mingled extrusive 
and sedimentary rocks between the original crust and the domi- 
nantly sedimentary formations above. (3) On the planetesimal 
theory, the oldest rocks which we might hope to reach would be 
those referred to the Extrusive Eon (p. 313, zone 3, Fig. 293), during 
which more or less sedimentary rock was mingled with the volcanic. 
On this hypothesis, as on the preceding, the line of demarkation 
between dominantly sedimentary rocks above, and dominantly 
non-sedimentary rocks below, would not be sharp. 

The rock-formations now most widely exposed at the surface are | 
sedimentary, and were formed during the Gradational Eon (p. 313). 
In many places, however, diverse formations which are predomi- 
nantly extrusive (igneous or meta-igneous) are found, either beneath 
the prevailing sedimentary rocks, or projecting up through them in 
such relations as to show their greater age (Fig. 302). In many 
cases these lower and older rocks were thoroughly metamorphosed, 
and in essentially their present condition, before the deposition of 
the overlying beds. These dominantly igneous and meta-igneous 
formations, older than the oldest known dominantly sedimentary 
rocks, are the oldest formations known, and the era during which 
they were formed is the first era of hich there is definite record i in 
the accessible formations of the earth. 


THE ARCHEAN ROCKS 317 


This lowest and oldest group of rocks is very complex, embracing 
lava flows, volcanic tuffs, igneous intrusions and sedimentary rocks, 
all more or less metamorphosed and deformed. Distinct fossils 
have not been found in them, but the presence locally of (1) carbo- 
naceous slates similar to younger slates containing carbon of organic 
origin, and (2) occasional formations of limestone and chert, are 
thought to imply the existence of life, and to warrant placing the 
era when these rocks were formed in the zoic group of eras (p. 313). 
The time during which, or during the later part of which, this oldest 
system of accessible rocks was made, is the Archeozoic era. 

Under the planetesimal hypothesis, the oldest known rocks may 
be referred confidently to the Archeozoic era, for, according to this 
hypothesis, rocks of organic origin and rocks containing organic 
products were not only mingled with all series that are accessible, 
but with great thicknesses of rock below, since life is supposed to 
have originated long before the earth acquired its present size. 
The oldest formations known also may be Archeozoic under the 
modified form of the nebular hypothesis (Fig. 292); but under the 
original form of the hypothesis, the original crust cannot be Archeo- 
zoic, since it antedated life. The term Archean (Archean system, 
Archean complex) is applied to the formations here referred to the 
Archeozoic era. ‘This term is applied to the oldest group of accessi- 
ble rocks, whatever their origin, and whether contemporaneous with 
life or antedating it. 

Delimitations of the Archean. The bottom of the Archean sys- 
tem is assumed to be inaccessible. Its upper limit has been fixed 
differently by different investigators. As first defined, the Archean 
(very old) included all rocks below the Cambrian (p. 323); but 
later it became clear that the rocks below the Cambrian should be 
differentiated into two great groups, the upper of which consists of 
several great systems of dominantly sedimentary or meta-sedi- 
mentary rocks, unconformable with one another, while the lower 
is dominantly igneous and meta-igneous. The term Archean is now 
generally restricted to the latter. The upper limit of the Archean 
is therefore the base of the oldest dominantly sedimentary system. 


GENERAL CHARACTERISTICS OF THE ARCHEAN! 
As now defined, the Archean includes two great classes of forma- 
tions, (1) a great schist series, and (2) a great granitoid series. 


‘Van Hise and Leith, Mono, LII. U.S. G, S, Chapter XX, and 16th Ann. Rept. 
U.S, G.S., Pt. I. pp. 744-759. 


318 ARCHEOZOIC ERA 


(1) The schist series is made up chiefly of the metamorphosed 
products of lava flows and volcanic tuffs. In composition they vary 
greatly, but the dominant types are hornblende schists, other green- 
stone schists, and mica schists. Associated with the metamorphosed 
surface lavas and pyroclastic formations, there are some massive 
igneous rocks, and occasional beds of metamorphosed conglomerate, 
sandstone, shale and limestone, and beds of iron ore, all of which 
imply the contemporaneous activity of water. 

(2) The granitoid series. One of the conspicuous features of 
the Archean system, in its present eroded condition, is the great 
masses of granite and gneiss that protrude through the schists. 
Formerly, these granites and gneisses were regarded as the oldest 
rocks, and were styled ‘‘primitive” or ‘‘fundamental”’; but it is 
now known that many of them, at least, are intrusions into the 
schists, and therefore younger than the latter. The gneisses are 
regarded as metamorphosed granites. 

In the formation both of the surface flows and the intrusions, 
the ascending lavas must have occupied numerous fissures or con- 
duits connected with the interior; hence there are numerous dikes 
and other intrusions, traversing the older parts of the Archean. 
It is to be borne in mind also that all younger intrusions and extru- 
sions of lava must have passed through the Archean, leaving in- 
trusions of diverse sorts (p. 228). These later intrusions are not 
strictly a part of the Archean, but they are not always separable, and 
their presence adds to the complexity of the Archean as a whole. 

Diastrophism and metamorphism. The most satisfactory ex- 
planation of the prevalent foliated structure of the Archean (Fig. 
294) is that which refers it to the movements of the outer part of 
the earth in Archeozoic and later time. Intrusions of igneous rock 
probably aided metamorphism (1) by furnishing heat, and (2) by de- 
veloping pressure. ‘The pressure was developed in two ways, (a) by 
the intrusion itself, which developed pressure when it was intruded, 
and (0) the shifting of so much lava from below upward must have 
caused the outer parts of the earth to settle down to take the place 
of the material transferred upward. 

That the rocks should have been much metamorphosed under 
these conditions is natural. By crushing and shearing, massive 
igneous rocks were given a foliated or schistose structure, and it is 
in the rocks of this era especially that metamorphism of this type 
prevails. It is now believed that the larger part of existing gneisses, 


THE ARCHEAN ROCKS 319 





Fig. 294. Metamorphic rock, showing foliation distinctly; bank of the Ottawa 
River. (Ells.) 


as well as a considerable part of existing schists, got their foliated 
structure in this way; but it is to be understood that some of the 
schists and perhaps some of the gneisses arose from sedimentary 
formations in other ways. It is not to be understood that the 
metamorphism of the Archean rocks was completed during the 
Archean era. ‘The metamorphosing processes of subsequent times 
have affected them. 

It would be difficult to obtain an exaggerated idea of the com- 
plexity of the rocks which has caused this system to be called a 
‘“complex.”” It consists in some places of rocks which are mainly 
massive (igneous intrusions); in other places, of rocks which are 
mainly gneissic (chiefly meta-igneous); and in still others, of rocks 
(largely meta-igneous and subordinately meta-sedimentary) in 
which a schistose structure predominates. Furthermore, the rocks 
of each of these structural types have a wide range in composition, 
from acid on the one hand to basic on the other. Rocks of all these 
classes are intimately associated locally, and any one may pre- 
dominate over the others. In places the rocks of the several struc- 


oor ARCHEOZOIC ERA 


tural types graduate into one another so completely as to leave no 
line of separation, while in others their definition is sharp. Thus 
massive rock appears in distinct dikes in gneisses and schists in 
some places, while in others schists are in dike-like sheets in rocks 
which are more massive. Furthermore, the relations of these sev- 
eral sorts of rock have been complicated greatly by the distortion 
to which they have been subject. The structure and relations of 
the several sorts of rock in the system indicate that it was (1) by 
successive intrusions, large and small, of rocks of different chemical 
composition into (2) still older rocks which were originally (a) 
chiefly extrusive-igneous and of varying chemical composition, but 
(b) subordinately sedimentary; and (3) by successive dynamic 
movements resulting in various degrees of metamorphism and de- 
formation of the various parts, that the intricate structure and 
composition of the Archean complex was attained. 

Though the variations in the rocks of the Archean system are 
great, there is yet a certain homogeneity in the heterogeneity of the 
whole. No large part of the system is very different from any other 
large part, and no definite and orderly relationship between the 
different parts has been made out over any considerable area. 
There appears to be no traceable succession of beds, and no definite 
stratigraphic sequence, such as can be made out in great series of 
younger meta-sedimentary rocks. 

Earlier views concerning the Archean. In explanation of the Archean 
system, many hypotheses have been suggested at one time and another, most of 
them starting with the Laplacian hypothesis as a beginning. One of them is that 
the Archean rocks are wholly of metamorphosed sediments, a second, that they are 
igneous rocks produced by the fusion of sediments, and a third, that they are 
igneous rocks intruded beneath the oldest sedimentary rocks after the deposition 


of the latter. These hypotheses have historic interest, but are not now generally 
held by geologists.! 


DISTRIBUTION 


In speaking of the distribution of a formation, its distribution 
at the surface generally is meant, and in speaking of its surface 
distribution, the mantle rock (glacial drift, etc.) which overlies 
and conceals it is usually ignored unless it is so thick as to make 
the underlying formation indeterminable. When the surface dis- 
tribution of the formation is given, therefore, it is not to be under- 
stood that the formation is literally at the surface everywhere within 
the area specified, but rather that it is exposed here and there within 

1 See the authors’ Earth History, Vol. II, 


THE ARCHEAN ROCKS 225 


that area, and that between the points of exposure it is the upper- 
most formation beneath the mantle rock. In this sense, the Archean 
rocks are estimated to appear at the surface over about one-fifth of 
the area of the land; but since great areas in some continents have 
been reconnoitered only, geologically speaking, this figure is only 
a rough estimate. 

Concerning the real, as distinct from the surface distribution, the 
Archean is the one accessible rock system which, theoretically, 
envelops the globe completely. No later system does this, for 
wherever the Archean comes to the surface, later formations are 
necessarily absent. 

In North America,‘ by far the largest area of Archean rock is 
in Canada (Fig. 295). Formations of the Proterozoic and later 
eraS occupy numerous small tracts within the area shown on the 
map, though the Archean underlies them at no great depth. Lying 
rudely parallel to the great Canadian area on the southeast is 
an interrupted series of probably-Archean tracts, extending from 
Newfoundland to Alabama. Similarly, on the southwest, there is 
a belt of detached areas stretching from Mexico to Alaska. In few 
places within these belts have the ancient rocks been studied in 
detail. Lesser areas of Archean rock appear in northern Michigan, 
Wisconsin, and Minnesota, and in the Adirondack region, but in 
some of these places, Archeozoic rocks have not been carefully 
separated from Proterozoic. The vicinity of Lake Superior in 
Canada, Michigan, Wisconsin, and Minnesota, the area north of 
Lake Huron, and the Ottawa region in Ontario, are the areas where 
the system is best known. 

The Archean system contains some iron ore (p. 332) and some 
ores of other metals, but not as a rule of great richness. Gold 
especially is widespread ”, but in few places is it known to occur in 
workable quantities. 

In other countries, the general characters and relations of the 
Archean of North America seem to be duplicated. A corresponding 
system of rocks, made up primarily of meta-igneous rocks, but 
subordinately of meta-sedimentary rocks inextricably involved 
with them, is known in all continents. The general characteristics 
and relations of the Archean therefore appear to be world-wide. 


1Van Hise, Pt. II, 16th Ann. Rept., U. S. Geol. Surv., pp. 744-843, and 
Van Hise & Leith, Monograph LII. 
2 Op. Cit., p. 295. 


322 ARCHEOZOIC ERA 





Fig. 295. The white areas north of Mexico represent exposures of Archean; those 
of Mexico represent lack of knowledge. The black areas represent exposures of Pro- 
terozoic, and the lined areas represent Archean beneath later formations. The light 
shading about the borders of the land represents the continental shelves, or, what is 
the same thing, the area of the epicontinental seas for this continent. 


TIME DIVISIONS 


323 
GENERAL TABLE OF GEOLOGIC TIME DIVISIONS! 
aed Present 
: Pleistocene 
Cenozoic ¢ Pliocene 
| Miocene 
Oligocene 
Eocene 
Transition 
( Cretaceous (Upper Cretaceous) 
Mesozoic Comanchean, or Shastan (Lower Cretaceous) 
Jurassic 
Triassic 
Permian 
Pennsylvanian (Coal Measures) 
Wide-spread unconformity 
Mississippian (Subcarboniferous) 
Paleozoic Devonian 
Silurian 
Wide-spread unconformity 
Ordovician 
Cambrian 
Great unconformity 
[ Keweenawan 
Unconformity 
Upper Huronian (Animikean) 
Proterozoic Unconformity 
Middle-Huronian 
Unconformity 
| Lower Huronian 
Great unconformity 
Great Granitoid Series 
(Intrusive in the main; 
Laurentian) 
Archeozoic ) Archean Complex + Great Schist Series 
(Mona, Kitchi, Keewatin, 
’ Quinnissee; Lower Huronian 
of some authors) 


THEORETICAL CONSIDERATIONS 


Bearing on theories of the earth’s origin. With the essential 
facts concerning the constitution and structure of the Archean in 
mind, it is in order to inquire to what hypothesis of the earth’s 


1 There are many unconformities not suggested in the table, where only those 
which appear to be extensive are noted. Those between the systems of the Pro- 
terozoic are known to be general for the Lake Superior region only. 


324 - ARCHEOZOIC ERA 


origin they best adjust themselves. The constitution of the system 
makes it clear that it does not represent the original crust of the earth 
or its downward extension. Jt cannot be affirmed, however, that 
no part of what is now classed as Archean is referable to an original 
crust; that is, it cannot be affirmed that no part of the Archean is 
referable to an azoic or pre-zoic period, strong as the evidence 
against such reference may seem. On the other hand, all the facts 
now known concerning the Archean adjust themselves to the 
planetesimal hypothesis, or to the modified form of the gaseo-molten 
hypothesis. They cannot, however, be said to establish either, or 
to preclude other hypotheses of the origin of the earth. 

Life. The presence in the Archean system of carbonaceous 
material and of limestones, seems to imply the presence of life 
during the era of its formation. Since no fossils have been found, 
nothing is known of the character of the life, and little, except by 
inference, of its abundance. 

Duration of the era. Of the duration of the Archeozoic era 
nothing can be said beyond the general statement that it was very 
great, a conclusion which is independent of any theory of the 
earth’s origin. If the planetesimal hypothesis is correct, there is 
no readily assignable lower limit to the Archean system, and the 
duration of the Archeozoic era may exceed that of all subsequent 
time. 

Climate. Nothing is known of the climate of the era except 
that :t seems to have been such as to permit the existence of life, 
and the ordinary phases of sedimentation. 


CHAPTER XIV 
THE PROTEROZOIC ERA! 


FORMATIONS AND PHYSICAL HISTORY? 


The time between the Archeozoic era and the deposition of the 
oldest system (the Cambrian) of rocks containing abundant fossils 
constitutes the Proterozoic era. It was during this era that sedi- 
mentation first became the leading process in the formation of the 
geological record. The composition of the sediments, now indura- 
ted, implies mature weathering, and their extent and thickness 
imply the prolonged deposition on low lands or in the sea, of the 
‘ sediments which were the products of mature weathering. During 
the era several great systems of sedimentary formations were 
formed. With the sedimentary formations there is much igneous 
rock, some of which is intrusive and some extrusive. 

Stratigraphic relations of the Proterozoic rocks. Great uncon- 
formities separate the Proterozoic rocks from the Archean below 
and the Paleozoic above. Great unconformities usually involve 
three elements: (1) a change in the attitude of the lower formation, 
as the result of which it is subject to erosion; (2) a long period dur- 
ing which its surface is eroded; and (3) the deposition of the over- 
lying rocks on the eroded surface. 

A sequence of events which might have given rise to. the uncon- 
formable relations of the Archean and Proterozoic is illustrated by 
Figs. 296 and 297. Fig. 296 represents an area of land composed 
of Archean rock, subject to erosion. The sediments derived from 
it are deposited in the sea (ata). In Fig. 297, the land is represented 
as having sunk so as.to be mostly submerged. -. Sediments (Al) 
washed down from the remaining land are being deposited uncon- 
formably on the eroded surface of ®. Though widespread, the 


1 Proterozoic, as here used, is a synonym for Algonktan as used by the U. S. 
Geol. Surv. 

2 A review of the pre-Cambrian geology of North America, by Van Hise and 
Leith, is found in Bull. 360, U.S. Geol. Surv., 1909. This Bulletin suggests probable 
correlations of the pre-Cambrian of different regions, so far as now warranted. 


325 


326 PROTEROZOIC ERA 


unconformity between the Archean and the Proterozoic is probably 
not universal, for there are doubtless places where the surface of the 
Archean did not suffer notable erosion before the deposition of 
Proterozoic sediments upon it. 





Fig. 296. Diagram showing Archean land (A) with sedimentation, a, along its 
vorders. (Compare Fig. 297.) 





Fig. 297. Diagram representing the same region as Fig. 296, after subsidence: 
The a of this figure corresponds to a of Fig. 296. 


Subdivisions. No existing classification of the Proterozoic 
formations has general application, but in the Lake Superior region, 
where these rocks are best known, four great unconformable systems 
are referred to this era. In some other regions the number is three 
(Fig. 298), in others two, and in still others but one. In most places 
each system is thousands of feet thick. These thick systems of 





Fig. 298. Diagram showing Proterozoic where it is composed of three systems 
of rock in the Lake Superior region. H,Huronian; 4, Animikean; K, Keweenawan. 
The diagram also shows the relation of these Proterozoic systems to the Archean 
(€R) below and to the Cambrian (€) above. The cross-pattern represents igneous 
rock. The lines, dots, etc., above the Archean represent sedimentary beds. 


HURONIAN SEDIMENTATION 327 


rock and the unconformities between them record the history of 
the era for this region. 

Proterozoic sedimentation. The surface of the land on which 
the Proterozoic sediments were deposited was probably comparable 
to existing land surfaces of crystalline rock which have been long 
exposed to weathering and other phases of erosion. The topog- 
_ raphy was doubtless more or less uneven, and the surface mantled 
by soil and residual earths and rock debris (mantle rock) which had 
arisen from the decay of the underlying rock. The general nature 
of the clastic sediments laid down on such a surface when it became 
an area of deposition are readily inferred. They were made up 
chiefly of (1) the disintegrated products already on the surface, (2) 
the materials worn from the rocks by waves, if the surface was 
covered by the sea, and (3) river detritus. 

1. One of the first effects of the Proterozoic seas, as they slowly 
transgressed the land — for it is presumed that this transgression 
was slow — was to work over, assort, and re-deposit the loose 
material on the surface. The coarse sediments were left in the 
shallow waters, while the fine materials were carrjed farther from 
shore, and left in the more quiet waters beyond. Deposits of gravel, 
sand, and mud were doubtless being made at the same time in 
different places, and changes in the position of the shore line, and 
in the depth of water, brought about, in time, the deposition of fine 
sediment on coarse, and of coarse sediment on fine. Thus the sedi- 


Sea Leve/ 





Fig. 299. Diagrammatic section showing relations which are conceived to 
have existed around Archean lands early in the early Proterozoic. Huronian 
sediments (A/) are in process of deposition. They are affected by intrusions and 
extrusions of lava, d1, do, d3, etc. 


mentary deposits came to be arranged in beds of different sorts, 
coarser and finer alternating in vertical section, and grading into 
each other laterally. 

At the base of the Proterozoic there is a widespread formation 
of conglomerate (Fig. 299) which appears to be composed of the 


328 PROTEROZOIC ERA 


coarse parts of the mantle rock which were on the surface when the 
Proterozoic seas transgressed the lands of Archean rock. Such a 
formation is known as a basal conglomerate, and is one of the best 
indices of an unconformity. 

2. Besides working over the decayed rock, the waves doubtless 
attacked the solid rock wherever exposures were favorable. The 
sediments thus acquired resembled the parent formation in average 
composition, and are thus distinguished from those of the preceding 
class, which were the products of rock decay. 

3. Streams descending from the land must have brought down 
gravel, sand, and mud. The larger part of the river-borne detritus 
was probably decomposed rock, but a smaller part was doubtless 
derived by the mechanical action of running water on undecayed 
rock. Once in the sea, these several sorts of detritus were mingled. 

Since some of the constituents (especially alkalies and alkaline 
earths) of the Archean rock dissolved during the processes of de- 
composition probably remained in solution in the sea-water, it is 
thought that the clastic sediments were more siliceous than the 
rock from which they were derived. 

The sorting power of moving water takes account of the physical 
characteristics of the material handled, and not of their chemical 
constitution; but in the decomposition of Archean rock, the quartz 
remaining in the residual mantle was generally in larger particles 
than the clayey matter derived from the silicates, and under the 
sorting influence of the waves the quartz grains (sand) were more or 
less completely separated from the clayey parts (mud). Thus 
materials which were unlike chemically were separated from one 
another because they were unlike physically. If the Proterozoic 
seas had abundant life which secreted calcium carbonate, or if their 
waters anywhere became overcharged with calcium carbonate, lime- 
stone might have been formed. 

Extent. Sediments accumulated in the Proterozoic era are 
known in limited areas only, but doubtless they were very wide- 
spread. Water-borne and wind-blown sediment must have reached 
all parts of the sea, and the life of the salt water probably made 
deposits over the whole of the ocean bottom. Some sediments, too, 
must have been left on land, as at all other stages of the earth’s 
history since sedimentation began. 

The exposed formations. ‘The sedimentary beds of the Protero- 
zoic consist of conglomerates, sandstones, shales, and limestones, 


HURONIAN FORMATIONS 320 


or their metamorphic’ equivalents. Before being cemented or 
otherwise solidified into firm rock, their materials were gravel, sand, 
mud, etc. The manner in which such materials are derived from 
older formations and transported to places of deposition, has been 
explained in earlier chapters. 

Basal conglomerate is of common occurrence at the bases of the 
several systems of the Proterozoic. There are also conglomerate 





Fig. 300. Section of the Proterozoic at a point in northern Michigan. (2), 
Archean granite. The other formations are Proterozoic. Length of section, 3 
miles. (U.S. Geol. Surv.) 





Fig. 301. Section showing the complex structure of the Archean and Proterozoic 
formations at one point in the Marquette (N. Mich.) region. A gr, Archean gran- 
ite. The other formations are Proterozoic. Length of section, 2 miles. (U. S. 
Geol. Surv.) 


beds which are not basal, and they point to changes in the condi- 
tions of sedimentation even where unconformities were not de- 
veloped. Quaritzite, composed chiefly of grains of quartz firmly 
cemented, occurs in thick and extensive beds. The quartz grains 
probably came from granitic rocks, and their separation from the 
other materials indicates the thorough decomposition of the rock, 
and ample opportunity for the rolling and rounding of the grains 
before they came to rest. As the quartzites of the Proterozoic are 
thousands of feet thick in some places, great bodies of rock must 
have been decomposed to furnish so much sand. There are also 
great beds of shales, or their metamorphic equivalents, which are 
interpreted as the clayey products of the decomposition which set 
the quartz free. Limestone is present, from which it is inferred that 
the sea had become calcareous by processes similar to those now in 
operation, and that a portion of the calcareous content of the 
waters was extracted and deposited. 

The inference that these ancient sediments were deposited in 
the same manner as sediments of modern times is supported by 


330 PROTEROZOIC ERA 


the ripple- and other shallow-water marks on the surfaces of the 
layers, and by their lamination and stratification, all of which are 
similar to those of sediments now being deposited. 

Geographic relations of exposed Proterozoic and Archean. 
Proterozoic rocks appear at the surface in many parts of North 
America, but they have been clearly separated from the Archean in 





Fig. 302. Diagram showing a common surface relationship between Archean 
(AR), Proterozoic (Al), and Cambrian (€). The Proterozoic formations appear at 
the surface between younger and older formations. 


few regions. Fig. 295 shows the area where rocks of known Proter-. 
ozoic age lie at the surface, together with areas where they have not 
been differentiated from the Archean. In many places, the Protero- 
zoic rocks at the surface are near areas of exposed Archean. 

That the Proterozoic formations should be exposed most com- 
monly about the borders of the Archean is made clear by Fig. 302, 
which shows, in section, the 
general relations of the Prote- 
rozoic systems (A/) to the 
Archean (&) below, and to 
younger formations (€) above. 
The same relations are shown 
in ground-plan in Fig. 303. 
While the relations shown in 
these diagrams are common, 
there are areas of Archean not 
surrounded or bordered by ex- 
posed Proterozoic formations, 
and areas of the latter not as- 
sociated with exposed Archean. Various relations of the two are 
illustrated by Figs. 304 and 305. 








Fig. 303. Map of the formations 
shown in section in Fig. 302. 


It is to be borne in mind that the map (Fig. 295) shows only the exposed areas 
(as now known) of Archean and Proterozoic. The Archean is presumably uni- 
versal, beneath other formations. The Proterozoic is not universal, but its extent 
is much greater than the area where it appears at the surface, Thus the Proterozoic 


THE HURONIAN SYSTEMS 331 


of Wisconsin is probably continuous beneath younger formations with the Pro- 
terozoic of southwestern Minnesota, the Black Hills, and the Rocky Mountains on 
the west, and with that of Missouri and Texas on the south. 





Fig. 304. Diagram showing how Proterozoic rock (Al) may fail to outcrop 
about Archean (4). 





Fig. 305. Diagram showing how Proterozoic rock (A/) may outcrop on one 
side of an area of Archean (4) and not on the other. 


THE PROTEROZOIC OF THE LAKE SUPERIOR REGION ! 


The Proterozoic formations have been most carefully studied 
and their relations are best understood in the region about Lake 
Superior, and the formations of this region have become, in some 
measure, the standard of comparison for the Proterozoic group as a 
whole. The four great unconformable systems, their relations to 
one another, to the Archean below, and to the Cambrian above, 
are as follows: 


Earliest Paleozoic Cambrian 
Unconformity 
4. Keweenawan 
Unconformity 
3. Upper Huronian (or Animikean) 
Proterozoic Unconformity 
2. Middle Huronian 
| Unconformity 
[ 1. Lower Huronian 
Unconformity 
Archeozoic Archean 


The Huronian Systems 


The first three systems of the Proterozoic group have much in 
common. All are dominantly sedimentary, and each includes 
formations of the common sorts of clastic rock or their metamor- 
phosed equivalents, together with limestone and beds of iron ore. 


1 Van Hise and Leith. Mono. LII, U. S. Geol. Surv. 
2 Jour. Geol. XIII, p. 161. 


332 PROTEROZOIC ERA 


Since none of the limestones are known to contain fossils, their 
organic origin cannot be affirmed. Each of the three periods of 
sedimentation was long, though their duration is unmeasured. 
Each system contains much igneous rock, some of which was 
extruded while sedimentation was in progress, and some intruded 
later. Locally, igneous rock is more abundant than sedimentary. 
The unconformable relations of the three Huronian systems, and 
the unconformity of the third below the Keweenawan, show that 
after the deposition of the first, second, and third systems respective- 
ly, geographic changes occurred, resulting in erosion where sedi- 
mentation had been in progress. 

The material for the sedimentary part of the first of these 
systems doubtless came from the exposed part of the Archean, 
while the sedimentary parts of the second and third systems came 
from the exposed parts of all older formations. : 

In places, the sedimentary rocks still remain in the condition 
of conglomerate, sandstone, and shale, though more commonly the 
sandstone has been changed to quartzite or quartz schist, and the 
shale to slate or schist. Some of the igneous rock is massive, while 
some of it has been changed to schist. The rocks which are least 
altered are, as a rule, those which have been least deformed, and 
in places they are still nearly horizontal, as when first deposited. 
The oldest system is, on the average, most metamorphosed, and the 
youngest least. 

Carbonaceous slates. One of the significant formations of this 
region is black shale or slate, whose color is due to carbon. The 
carbon is thought to imply the existence of life when the sediments 
were deposited. Where the rocks are highly metamorphic, the 
black shale has been changed to graphitic schist. 

Iron ore. Another important formation is iron ore. Here 
belong the iron ores of the Mesabi (Minn.), Penokee-Gogebic (Wis. 
and Mich.), Menominee (chiefly Mich.) and other regions (Fig. 306). 
The ore is in the form of ferric oxide (chiefly hematite, Fe.O3), but 
in this form it represents an alteration from an iron-bearing forma- 
tion, originally deposited as chemical sediments, composed largely of 
iron carbonate and iron silicate, with some ferric oxides. These 


materials are believed to have been derived, directly or indirectly, 


from basic igneous rocks, extruded into the sea.!. The alteration 
to ore was brought about at a later time, by ground-water circulat- 
ing through the rocks. 

1 Van Hise and Leith. Mono. LII, U. S. Geol. Surv. 


HURONIAN IRON ORES 333 


The region about Lake Superior yields more iron ore than any 
other area of equal size in the world. In 1913 the aggregate pro- 
duction of this region was about 50,000,000 long tons,! which was 
about 83 per cent of all that was produced in the United States that 


a as 


u. 
ene 





Fig. 306. Map showing (in black) the position of the iron-producing areas in 
the Lake Superior region. 1, Michipicoten district; 2, Kaministikwia and Matawin 
district; 3, Steep Rock Lake and Attikokan district; 4, Vermilion district; 5, Mesabi 
district; 6, Penokee-Gogebic district; 7, 8, and 9, Marquette, Crystal Falls, and Me- 
nominee districts. 
year; of this, the Mesabi region produced nearly 34,000,000 tons. 


The ores of the Lake Superior region are partly in the Archean 
(about Vermilion, Minn.), partly in the older divisions of the Hur- 
onian group (about Marquette, Mich.), but most largely in the 
Animikean. ‘The following table! gives the production in tons 
for the principal areas for certain years preceding 1911: 





1890 1895 Ig00 1905 IQIo 
Marquette...... 2,863,848 1,982,080 | 3,945,068 | 3,772,645 | 4,631,427 
Menominee..... 2,274,192 | 1,794,970 | 3,680,738 | 4,472,630 | 4,983,729 
PiOeevigge..... - 2,914,081 | 2,625,475 | 3,104,033 | 3,344,551 | 4,746,818 
MermnOniws =. . 891,910 | 1,027,103 1,675,049 1,578,626 | 1,390,360 
COO Oe ee 2,839,350 | 8,148,450 | 20,156,566 | 30,576,400 
POROUS bec). 8,944,031 ! 10,268,978 ! 20,564,238 | 33,325,018 | 46,328,743 


1 Mineral Resources of the United States, 


334 PROTEROZOIC ERA 


Other ores.! Silver, nickel, and cobalt occur in workable quan- 
tities in the Huronian rocks at various points, especially in Canada. 
Rich ores of silver and cobalt (largely Lower Huronian) are found 
at Cobalt, Ontario, and ores of nickel at Sudbury. 

Thickness. ‘The thickness of the Huronian systems is hard to 
measure, because of their deformation; but if the maximum thick- 
ness of the individual formations of different localities is taken, their 
aggregate is several miles. 

The following section from the Marquette region may be regard- 
ed as fairly typical for the region: 

Michigamme slate and schist. Several thousand feet 
(maximum) in thickness. 


Ishpeming formation, largely quartzite. 1,500 feet (max- 


Upper Huronian - 
| imum) thick. 


Negaunee formation or series (slate, schist, Jaspilite, iron 
ore, etc.). 1,500 feet (maximum) thick. 
Middle Huronian ~ Siamo slate. 1,200 feet (maximum) thick. 
Ajibik quartzite (in places schistose). Nearly 1,000 feet 
| (maximum) thick. 


than 1,000 feet (maximum) thick. 

Kona dolomite (some clastic beds). More than 1,300 feet 
(maximum) thick. 

Mesnard quartzite. Several hundred feet (maximum) 
thick. 


| Wewe slate (including some other sorts of rock). More 


Lower Huronian | 


The Keweenawan System 


Constitution and thickness. In some parts of the Lake Superior 
region a fourth system of pre-Cambrian rocks, the Keweenawan, 
overlies the Upper Huronian. Unlike the Huronian systems, it is 
composed more largely of lava-flows than of sedimentary strata. 

The lava beds of the Keweenawan constitute its lower and 
larger part. The earlier flows of lava seem to have occurred on 
land, and to have followed one another at short intervals, for the 
surface of one flow was not eroded much before the next overspread 
it. Later, the intervals between flows appear to have been longer, 
and thin beds of sediment were deposited between successive sheets 
of igneous rock. The sedimentary beds increase in importance 
upward until, in the upper part of the system, lava beds fail alto- 
gether. In the valley of the St. Croix River, in northwestern Wis- 
consin and the adjacent parts of Minnesota, there are said to be 65 

1Van Hise and Leith. Monograph LII, U. S. G. S., pp. 591-6. 


THE KEWEENAWAN SYSTEM 335 


lava-flows and 5 conglomerate beds in succession, with neither top 
nor bottom of the system exposed. 

The igneous rocks of the system consist principally of gabbros, 
diabases, and porphyries; but other varieties are also present. The 
sedimentary rocks, chiefly sandstone and conglomerate, were de- 
rived largely from the igneous, and their character is such as to 
indicate that they accumulated rapidly. The thickness of the 
sedimentary beds has been estimated at some 15,000 feet; but there 
is reason for questioning the interpretation of such figures. 

The total thickness of the Keweenawan system has been placed 
as high as 50,000 feet. Interpreted in the simplest way, this would 
seem to mean either that beds of rock were piled up nearly 1o miles 
high on land, or that they filled a basin some to miles deep. Since 
the upper part of the system is sedimentary, and sediments do not 





Fig. 307. Diagram of a series of beds formed on the abysmal slope of a conti- 
nent, or in some similar situation, showing that the thickness, as usually measured, 
ef, is not dependent on the depth of the basin, cd, and that a thick series does not 
necessarily imply subsidence, even when the exposed portions of it show evidences of 
shallow-water deposition at various horizons. 


accumulate in quantity in high places, the first of these suggestions 
cannot be entertained, and it is extremely unlikely that there ever 
was a basin ro miles deep. 

The thickness of great bodies of stratified rock is commonly 
measured as suggested by Fig. 307. The dip of the rock (p. 275) 
and its extent at the surface are measured, and the depth is then 
calculated on the principle that the thickness of the whole is equal 
to the thickness of all its parts. The thicknesses of the several 
beds, added together, is shown in the diagram by the line ef, whereas 
the actual thickness, from top to bottom, is shown by the line cd. 

The point may be illustrated in another way. On the outer 
slopes of continental shelves, and in deltas, sediments are laid down 
with a considerable angle of slope. If the Amazon were to build a 
delta out 200 miles, the present ocean bottom remaining at an 


336 PROTEROZOIC ERA 


average depth of four miles below the surface, and if the angle of 
deposition were 2°, the computed thickness of the deposits, accord- 
ing to the common methods of measurement, would be about 7 
miles. If the delta were built out 1,000 miles, the computed depth 
would be 35 miles, though the basin was but four miles deep. Ifa 
delta were built half-way across a lake basin too miles wide and 1,000 
feet deep, the angle of deposition being 3°, the thickness of the 
series, measured by the above method, would be 13,800 feet, though 
the basin was but 1,000 feet deep. With these points in mind it is 
clear that caution must be used in interpreting the great thicknesses 
sometimes assigned to sedimentary systems. 

The sedimentary part of the Keweenawan system has commonly 
been assumed to imply marine submergence; but so far as now 
known the sediments may have been accumulated in an interior 
basin, or may be partly subaérial. 








Fig. 308. Diagram illustrating the development of the Lake Superior syncline. 
AR, Archean; H and A, Huronian and Animikean; K, Keweenawan. (Irving, U. S. 
Geol. Surv.) 


Deformative movements. About the close of the Keweenawan 
period, the rocks of the system were somewhat deformed, and the 
deformation in the Lake Superior region was perhaps contempo- 
raneous with deformation in other parts of the continent. ‘These 
changes are regarded as marking the beginning of the end of the 
Proterozoic era. As a result of these deformations, some parts 
of the area where Keweenawan sediments had been deposited were 
brought into such an attitude as to be eroded, but the changes did 
not, as a rule, involve great folding or faulting of the strata. In 
keeping with their structure, the rocks are not greatly metamor- 
phosed. 

After the warping which followed the deposition of the Kewee- 
nawan system, the exposed surfaces of this and older systems suf- 
fered protracted erosion. Ultimately the land about Lake Superior 
sank again, and when the sea came back, a new series of sedimentary 
beds was deposited unconformably on the eroded surface of the 
older. The waters of the returning sea teemed with life, for the 
formation then made contains abundant fossils. This abundantly 


THE LAKE SUPERIOR PROTEROZOIC 337 


fossiliferous formation is a part of the Cambrian system, the oldest 
system of the Paleozoic group. 

Copper. The Keweenawan system contains the most extensive 
deposits of native copperknown. The metal occursin pores and cracks 
of the igneous rock, and between the pebbles and grains of some 
parts of the sedimentary beds. In the conglomerate at some of the 
richer mines, the copper is so abundant as to be an important 
cementing material of the rock. The copper is believed to have 
been deposited by magmatic waters (7. e., waters of lavas), and toa 
lesser extent by thermal ground waters which had dissolved the 
metal from the igneous and sedimentary rocks." 

In 1875 the Keweenawan formation of northern Michigan 
yielded 16,089 tons of copper, about 90 per cent of all that was 
produced in the United States. In 1911 the same area yielded 
109,093 tons, but this was only about 20 per cent of the copper 
produced in the country that year. 

The ores of silver, cobalt and nickel in the Huronian formations 
are to a large extent at least associated with basic igneous rocks, 
perhaps of Keweenawan age, intruded into Huronian rocks. 


General Considerations Concerning the Lake Superior Proterozoic 


Duration of time. It is difficult to conceive of the great lapse 
of time involved in the history of the Proterozoic era. The esti- 
mates give an aggregate thickness of more than 30,000 feet for the 
sedimentary rocks of the Proterozoic systems. The accumulation 
of so much sediment would in itself mean a vast lapse of time, and 
wher it is remembered that the four systems are separated from 
one another by unconformities, each of which may represent as 
much time as that involved in the accumulation of a system, it will 
be seen that the duration of the Proterozoic era was exceedingly long, 
possibly comparable to all succeeding time. It would appear that 
it should be spoken of in terms of tens (at least) of millions of 
years, rather than in terms of a lesser denomination. 

Destruction of rock implied. Thick beds of sediment mean the 
destruction of a still larger volume of older rock, for much of the 
more soluble part of the rock destroyed does not appear in the 
sedimentary formations. Had the Archean lands in the vicinity of 


1Van Hise and Leith, Mono. LII, U.S. Geol. Surv. For earlier discussions, 
see Irving and Van Hise, Mono. V, U. S. Geol. Surv., Chamberlin, Vol. I, Geol- 
ogy of Wisconsin. 


338 PROTEROZOIC ERA 


Lake Superior been high enough at any one time to furnish the thick 
sediments of the Proterozoic, their height would perhaps have sur- 
passed any existing elevation; but it is not probable that such ele- 
vations existed at any time. It is more probable that as erosion 
proceeded, the land reacted by rising slowly, or that the sea bottom 
sank, drawing off the waters and leaving the land relatively higher. 
In this way, degradation and elevation may have been in progress 
at the same time, and the one process may never have got far ahead 
of the other. The doctrine that the surface of the lithosphere sinks 
and rises under increase and decrease of load is one phase of the 
general theory of tsostasy. 

Succession of events. Reviewing the succession of events in 
the Lake Superior region, we find (1) that land composed of Archean 
rocks suffered prolonged erosion, but that the sites of the earliest post- 
Archean sedimentation areunknown. (2) Theland then sank or wasso 
eroded or deformed as to permit the deposition of the Lower Huro- 
nian sediments on parts of its eroded surface. (3) Areas including 
Archean and Lower Huronian rocks then came into such an attitude, 
presumably by crustal warping, that they were subject toa long period 
of erosion, with contemporaneous sedimentation elsewhere. Dur- 
ing the deformation, the rocks involved were somewhat meta- 
morphosed. (4) Again the land seems to have sunk, allowing the 
sea (conditions for deposition) to cover a large part of the area 
which had been subject to erosion just before, and to deposit upon 
its eroded surface the sediments of the Middle Huronian system. 
(5) After this long period of sedimentation, certain tracts seem to 
have emerged, exposing the landward border of the Middle Huronian 
system, and the older rocks not covered by it, to erosion. This 
emergence of areas of Middle Huronian sedimentary formations was 
accompanied by some deformation and metamorphism. (6) This 
period of erosion was followed by another period of submergence, 
when sediments (the Animikean) were laid down again in the Lake 
Superior region, this time on the eroded surface of the Middle 
Huronian or some older system. (7) Deformation, accompanied by 
emergence and followed by erosion, succeeded this third period of 
Proterozoic sedimentation. (8) Flows of lava of great magnitude 
were then poured out upon the surface of the land over consider- 
able areas, and intruded into older terranes. Before they ceased, 
sedimentation began again in the region, and soon predominated, 
the lavas and sediments making the Keweenawan system. 


THE LAKE SUPERIOR PROTEROZOIC 339 


(9) After the deposition of this system, much of it was exposed 
to erosion. 

This succession of events implies repeated changes of relative 
level of land and sea in the Lake Superior region during the era. 
We shall see that such changes are confined neither to this time nor 
to this region. Changes in the relations of sea and land are among 
the notable events of the earth’s history, even to the present time. 
Since many other changes are dependent on them, they are believed 
to furnish the best basis for the subdivision of geological history. 
It is not now possible to determine the extent of the crustal oscilla- 
tions which took place during this era; but enough is known of the 
extent of land in North America at the close of the Proterozoic to 
make its representation on maps instructive (the white areas north 
of Mexico, Fig. 295). 

Metamorphism. The lower rocks of the Proterozoic are, on the 
whole, more highly metamorphosed than those above, but the 
Animikean beds are locally as highly metamorphic as the Lower 
Huronian, indicating intense dynamic action, at least locally, after 
the deposition of the third great system. Since different sorts of 
rock behave differently under dynamic action, it follows that some 
beds are much more highly metamorphic than others associated 
with them, even though subjected to the same forces. 

There is scarcely a phase of metamorphism which the Protero- 
zoic rocks do not show. The schists, slates, and gneisses are espe- 
cially the product of dynamic metamorphism; the quartzites are 
the products of extreme consolidation by cementation; the iron ore 
is the product of aqueous metamorphism, effected by ground-waters, 
while other phases of metamorphism are due to the heat of intruded 
rock. It is not to be understood that the metamorphism of any 
considerable body of rock is effected by any one process alone. 
Dynamic action, which seems on the whole the most important 
factor in metamorphism, always generates heat, and high temper- 
ature, especially in the presence of water, facilitates chemical and 
mineralogical change. So, too, in the case of igneous intrusions, 
there may be great dynamic action as well as great heat, and water, 
an agent of chemical change, is always present. 

Events elsewhere. A series of events consonant but not neces- 
sarily identical with those of the Lake Superior region was probably 
in progress about every other area of Archean rock, during the 
Proterozoic era; but it does not follow that about every other 


340 PROTEROZOIC ERA 


Archean land area four great systems of rocks were laid down dur- 
ing this long era. About some such areas there may well have 
been one, two, or three systems of Proterozoic rocks instead of four, 
while about others, continuous sedimentation may have been in 
progress from the first of the Huronian periods to the end of the 
Keweenawan. 


PROTEROZOIC ROCKS IN OTHER REGIONS 


Pre-Cambrian sedimentary formations occur in many other 
parts of North America, in relations to the Archean similar to those 
already described. On the whole, they resemble the rocks of the 
Proterozoic systems about Lake Superior as closely as could be ex- 
pected under the general principles set forth. 

Some of the more important occurrences of Proterozoic rocks 
outside the Lake Superior region are the following: (1) in an exten- 
sive area north of the Great Lakes; (2) in the eastern provinces of 
Canada; (3) in the Adirondacks; (4) in isolated patches in the Mis- 
sissippi basin, in Wisconsin, northwestern Iowa and adjacent parts 
of Minnesota and South Dakota, in the Black Hills of South Dakota, 
in southeastern Missouri, and in Oklahoma; (5) in Texas; (6) in the 
Piedmont belt of the eastern part of the United States; and (7) at 
various points in the Cordilleras (Fig. 295). 

In some of these localities, the rocks are chiefly sedimentary or 
meta-sedimentary, while in others they are partly or even largely 
igneous. ‘Thus in the Black Hills, the Proterozoic rocks consist of 
slates, quartzites, schists, etc., intruded by granite. From the 
granite intrusions, the largest of which is eight or ten miles long 
and nearly as broad, numerous dikes penetrate the clastic beds, and 
furnish good illustrations of the metamorphosing effects of igneous 
intrusions. In the Adirondack region, pre-Cambrian rocks make 
up the larger part of the mountain mass. They include both:sedi- 
mentary (meta-sedimentary) and igneous rocks, the latter partly 
at least intrusive in the former. 

The Cordilleran region. The cores of many of the older moun- 
tain ranges of the west are believed to be of Archean rock. In 
many of them there are thick series of sedimentary or meta-sedi- 
mentary rocks (Proterozoic) overlying the Archean and surround- 
ing its outcrops, overlain in turn by Cambrian or younger strata. 
Sedimentary formations predominate among these Proterozoic 
formations, but are associated with igneous rocks which are in part 


PROTEROZOIC ROCKS IN THE EAST AND WEST 341 


contemporaneous. In most of these localities the Proterozoic rocks 
are unconformable beneath overlying formations, and above the 
Archean where that is shown. In much of the northwest, however, 
there is conformity between the Proterozoic and the Cambrian, 
according to present interpretations. 

In the Canyon of the Colorado, pre-Cambrian formations are 
well exposed. The Proterozoic (Grand Canyon) group, more than 
10,000 feet in thickness, rests unconformably on the Archean, and 
is in turn covered unconformably by the Cambrian. Here, as in 
Montana, a few fossils have been found. 

In the eastern part of the United States. There are large areas 
of metamorphic rock in the eastern part of the United States, former- 
ly classed as Archean. ‘Their position is shown in Fig. 295.° These 
metamorphic rocks include some that were sedimentary, and some 
that were igneous. A part of them are probably Proterozoic, but 
the Proterozoic, Archean, and metamorphic Paleozoic rocks have 
not been fully differentiated. 

Summary. While the correspondence of the Proterozoic rocks 
in these various regions with those of the Lake Superior region is 
not generally very close, it may be pointed out again that close 
correspondence is not to be expected, even if the rocks of different 
localities were contemporaneous in origin. The phases of sedi- | 
mentation taking place about any land mass at any time depend 
largely on the height of the land, the exposure of its coasts, climate, 
and the character of the formation suffering erosion. These various 
factors were as likely to be dissimilar as similar about the various 
centers of sedimentation. Igneous rocks form a not inconsiderable 
part of the Proterozoic systems, and there is no apparent reason 
why igneous activities in different regions should correspond either 
in time or in the nature of their products. Even deformations of 
the crust, which are the basis for the separation of the rocks into 
systems, need not have been the same in different regions. It fol- 
lows (1) that the number of Proterozoic systems bounded by uncon- 
formities may not be the same in all regions; (2) that the thick- 
nesses of the various systems may vary greatly; (3) that there 
need have been no close correspondence in the sorts of rock.in 
different regions at the outset; and (4) that they may have been 
metamorphosed unequally since their deposition. Dissimilarity of 
the Proterozoic in different regions was, therefore, to have been 
anticipated. 


342 PROTEROZOIC ERA 


Proterozoic Formations in other Continents 


Proterozoic formations are believed to exist in all continents. 
In more than one country where they have been studied, the pre- 
Cambrian sedimentary rocks are thought to belong to at least two 
unconformable systems. In Sweden, as about Lake Superior, iron 
ore occurs in these formations, and the great bodies of iron ore in 
Brazil probably are of similar age. 


LIFE DURING THE PROTEROZOIC ERA 


The presence of a few fossils in the Proterozoic rocks proves the 
existence of life during this era.!. The best-preserved fossils are 
arthropods (p. 686) resembling crustacea. There are-also tracks 
of two genera of worms. In addition, there are obscure forms which 
appear to be referable to brachiopods and pteropods. It is signifi- 
cant that the oldest definite fossils yet found are forms well up in 
the animal kingdom, and that they occur (in Montana) 9,000 feet 
below the unconformity between the Proterozoic and the Cambrian. 
Other lines of evidence indicating life are: (1) Carboniferous shales, 
slates, and schists, and (2) limestone, some of which occurs near 
the base of the Lower Huronian. This rock was formerly regarded 
as demonstrative of the existence of life; but in recent years the 
belief has gained ground that considerable formations of limestone 
may have originated by precipitation from sea-water. This origin 
is suspected for many limestone formations which are free from 
fossils, and if the hypothesis is applicable to any extensive formation 
of limestone, it may be applicable to that of the Proterozoic. But 
even without reliance on this sort of rock, the occasional fossils 
leave no doubt of the existence of life in this era. 


CLIMATE 


Since inferences concerning the climate of any period are drawn 
largely from fossils, and since fossils are exceedingly rare in the 
Proterozoic strata, they afford little warrant for conclusions con- 
cerning the climate of the era as a whole. Conglomerate beds 
which have been interpreted as glacial” are found at the base of 


1 For summary of knowledge concerning pre-Cambrian fossils, see Walcott, 
Bull. Geol. Soc. Am., Vol. 10, pp. 199-244. 

2 Coleman, Jour. Geol., Vol. XVI, pp. 149-158, and Wilson, Ibid., Vol. XXI, 
pp. 121-141. 


CLIMATE OF THE PROTEROZOIC 343 


the Proterozoic in the vicinity of Cobalt, Ontario. This inter- 
pretation, long doubted, now appears to be warranted. It may be 
noted that glacial formations are singularly out of harmony with 
the conceptions of the climate of early geologic time which have 
prevailed until recently. They are altogether in harmony with the 
conceptions of climate which grow out of the planetesimal theory. 


Map studies. Map studies should be carried on in connection with the chapters 
on the Archeozoic and Proterozoic. For this purpose, numerous folios of the U. S. 
Geological Survey are especially serviceable. See also Laboratory Exercises in 
Structural and Historical Geology, Salisbury and Trowbridge, Exercise VII. 


THE PALEOZOIC ERA 


CHAPTERS Aa 
THE CAMBRIAN PERIOD 


FORMATIONS AND PHYSICAL HISTORY 


The crustal movements which closed the Proterozoic era con- 
verted a large area within the limits of North America into land. 
This is shown by the distribution of the basal strata of the Cambrian, 
the oldest system of the Paleozoic era. Where accessible, the base 
of the system is, in most places, unconformable on underlying 
formations. The distribution of the successive parts of the system 
discloses the relations of sea and land throughout the period, for ~ 
most of the strata are of marine origin. 


Subdivisions 


The Cambrian system is divided into three parts, the Lower, 
the Middle, and the Upper. Georgian (Vt.), Acadian, and Potsdam 
or Saratogan (N. Y.), names of localities where the several divisions 
of Cambrian were first differentiated in North America, are syno- 
nyms for Lower, Middle, and Upper Cambrian respectively. The 
name St. Croixan (Wis.-Min.) also is used for the Upper Cambrian. 

Lower Cambrian. Lower Cambrian formations are known in 
North America only near the eastern and western borders of the 
continent (Fig. 309). In the east, they occur in the Appalachian 
belt and at some points farther east; in the west, they are found in 
various states between the rroth and the 120th meridians. Both 
east and west, the strata contain marine fossils. Those of the east 
were accumulated in straits, sounds, etc., rather than on the shores 
of the open sea. The great tract between the Appalachian Moun- 
tains on the one hand, and western Montana and Utah on the 
other, is believed to have been land during the early part of the 
period, and from it sediments were probably carried to the sea on 
either hand. 


344 


Stee eww en 


. 
‘ 
‘ 
‘ 
. 
‘ 
’ 


news, 


+ 


he ace Soe 





Fig. 309. Map showing the outcrops (in black) of Lower and Middle Cambrian 
tormations. The areas shaded by lines represent regions where the formations are 
believed to exist, though not exposed. The longer the lines, the better the basis 
for belief in the existence of the beds. The unshaded areas north of Mexico are 
believed to have been land during the early portion of the Cambrian period. The 
unshaded area south of the United States represents lack of knowledge. The 
shading within the area of the ocean is the same as in Fig. 295. The Middle Cam- 
brian may be somewhat more extensive than the map shows. The area not covered 


by the early Cambrian formations, and probably a still larger area, was land at the 
end of the Proterozoic, 


346 CAMBRIAN PERIOD 


Middle Cambrian. Strata of the Middle (Acadian) Cambrian 
are found above those of the Lower, and in addition in Texas, 
Oklahoma, Arizona, parts of Montana, and perhaps elsewhere. 
Since the Middle Cambrian beds contain marine fossils, their dis- 
tribution indicates that the continent was being invaded by the sea 
from the south and west before the close of the Middle Cambrian 
epoch. Middle Cambrian beds are absent from much of the inte- 
rior, if present identifications are correct. Where the Middle Cam- 
brian rests on the Lower, the two are generally conformable. Where 
the Middle overlaps the Lower, it is unconformable on older forma- 
tions. . 

Upper Cambrian. In the Later Cambrian (Potsdam, Saratogan, 
or St. Croixan) epoch, the sea overspread much more of the continent, 
for the Potsdam series covers not only the eastern and western bor- 
ders of the continent, but much of the interior as well. ‘The Upper 
Cambrian is, as a rule, conformable on the Middle in the east and 
west, but in the interior it is unconformable on pre-Cambrian 
formations. Fig. 310 shows something of the distribution of the 
Cambrian system as a whole. 


Basis for the Subdivisions 


We have now to inquire how the Cambrian system may be 
recognized, and further, the means by which the Lower, Middle, 
and Upper parts may be distinguished from one another. 

Superposition. Where a formation or series is conformable on 
another of known age, as the Middle Cambrian on the Lower, the 
presumption is that the upper was formed immediately after the 
lower. In this case, the approximate age of the upper is known. 
But where one formation is unconformable on another of known 
age, the stratigraphic relations between them do not show whether 
the upper is much or little younger than the lower. 

Fossils. ‘The Cambrian is the oldest system of rocks known to 
contain abundant fossils. Most of them represent the shells, other 
hard parts, or tracks of marine animals buried in the sands and muds 
when they were deposited. The fossils of any division of the 
Cambrian system constitute the known fauna of that stage, but it 
is not supposed that fossils of all species that lived have been 
preserved. . 

The Lower Cambrian formations contain certain fossils which 
are distinctive, Among them are species of a genus of trilobites 


FORMATIONS AND PHYSICAL HISTORY 347 





Fig. 310. Map showing the Upper Cambrian formations. The outcrops are 
shown in black. The continuous lines represent areas where the Upper Cambrian 
formations are confidently believed to exist, though concealed. |The dashes repre- 
sent areas where there is some reason for believing them to exist. The dotted areas 
represent areas from which the Upper Cambrian is believed to have been removed 
by erosion, The unshaded areas have the same meaning as in Fig. 309. 


348 CAMBRIAN PERIOD 


named Olenellus (Fig. 311, a). Along with representatives of this 
genus, many other species of various types are found. To the 
aggregate, the name Olenellus fauna is given, and Olenellus Cambrian 
is synonymous with Lower Cambrian and with Georgian. It is 
not to be understood that representatives of the genus Olenellus 





Fig. 311. CHARACTERISTIC CAMBRIAN TRILOBITES: @, Olenellus gilberti Meek; 
b, Paradoxides bohemicus Boeck; c, Dikellocephalus pepinensis Owen. These three 
genera are characteristic of the Lower, Middle and Upper Cambrian, respectively. 


are found in the Lower Cambrian everywhere, or that other genera 
of trilobites are absent. | 

Where formations representing the whole of the period are 
present, the fossils in the middle beds are not the same as those 
in the lower. At no single plane is there, as a rule, a striking change 
in species, but in successively higher beds some of the species found 
below disappear, and new species comein. ‘These changes show that 
the inhabitants of the sea changed as time went on. At about that 
stage in the Cambrian system where the genus Olenellus drops out, 
the genus Paradoxides (Fig. 311, 6) appears in some places. The 
species associated with Paradoxides are somewhat different from 
those associated with Olenellus. The Paradoxides and their asso- 
ciates constitute the Paradoxides fauna, which includes many 
species of other genera of trilobites, and many species not related 
to trilobites. By general agreement, the Middle Cambrian, on 
both sides of the North Atlantic, is defined by the Paradoxides 
fauna, so that Paradoxides Cambrian is synonymous with Middle 


FORMATIONS AND PHYSICAL HISTORY 349 


Cambrian and with Acadian (p. 344). In the western part of North 
America, and on the opposite side of the North Pacific as well, the 
Middle Cambrian does not contain Paradoxides. Its fauna is 
known as the Olenoides fauna, which is distinct from fauna of the 
Lower Cambrian. In like manner the Middle Cambrian fauna 
is succeeded by another, the Dzkellocephalus fauna found in the 
Upper Cambrian strata. Geologists have agreed to define the 
Upper Cambrian as the series of strata carrying this fauna. 

It is not to be understood that every species of the Paradoxides 
fauna is unlike every species of the faunas below and above. This 
is not the case; but so many species of the three faunas are different, 
that with a considerable number to judge from, their separation is 
possible by those familiar with Cambrian fossils. 

Sequence of faunas based on stratigraphy. The sequence of 
faunas was first determined by superposition of the strata. The 
Lower Cambrian fauna could not have been known to be older than 
the Middle Cambrian fauna if beds containing the former did not 
underlie beds containing the latter. In other words, the primary 
basis for correlation by means of fossils 1s stratigraphy. 


Physical Events of the Cambrian 


Submergence. The distribution of the several series of the 
system shows that the great physical event of the Cambrian period in 
North America was progressive submergence of the continent. Theo- 
rectically, this may have been brought about by a rise of the sea or 
by a lowering of the land, or by both together. Both the lowering 
of the land and the rise of the sea may be due to gradation, to dias- 
trophism, or to the two combined. 

Gradation as a cause of submergence. Gradation is perpetual 
and inevitable where land and sea exist. The waves attack the 
land along its borders, and the agents of land degradation lower 
its surface. The former is a direct cause of encroachment of sea 
on land, and the latter is an indirect cause, since all sediments car- 
ried from land to sea raise the surface of the sea correspondingly. 
Small as this rise is for any brief period, its effect is to cause the sea 
to advance on the land, and the lowering of the land by degradation 
at the same time increases the area of the advance. If continued 
long enough, shore-cutting about the borders of the lands, down- 
cutting over the whole surface, and the accompanying rise of the 
sea-level, must inevitably cause the water to cover the continents 


350 CAMBRIAN PERIOD 


provided there 1s no deformation of the body of the earth in the mean- 
time. 

If the earth were to remain without deformation long enough for 
the continents to be base-leveled, the deposition in the sea of the 
sediments thus derived would raise the water about 650 feet. This 
would submerge a large part of the base-leveled land. The evidence 
of gradation in the Cambrian period is clear and firm. Most of the 
sediments which make up the Cambrian system of rocks were 
eroded from the land and deposited in the sea. ‘This lowered the 
land and raised the sea. Gradation was, therefore, a factor in the 
submergence of the continent, and there is evidence that great 
progress was made toward base-leveling before the close of the 
period. 

If gradation were the sole agency involved in the submergence 
of the lands, the advance of the sea should have been steady, though 
not necessarily equal in rate at all times and places. Without 
going into details, it seems certain that there were changes in the 
areas of deposition other than those which can be accounted for by 
gradation, but none of these changes imply notable warpings such 
as are recorded in the rocks of the Proterozoic and Archeozoic eras. 

Deformation as a cause of submergence. Deformations which 
may cause submergence of land (and emergence of sea-bottom) are 
of various sorts. Any deformation which causes the land to sink, 
or the sea to rise, leads to submergence. Such movements and their 
causes have been discussed briefly (chapter VIII). One special 
phase of movement which may have especial significance here is 
noted at this point. | 

Continental creep. ‘The continents are about 15,o00 feet above 
the ocean bottom. Their weight causes an average pressure of 15,000 
to 20,000 pounds to the square inch on their bases, 15,000 feet down. 
This pressure tends to cause the continents to spread by creep into the ocean 
basins, on the same principle that an ice-sheet spreads. Spreading 
is opposed by the hydrostatic pressure of the oceans against the sides 
of the continental platforms. ‘This is some 5,000 pounds per square 
inch at the bottom, so that there remains an unbalanced pressure of 
10,000 to 15,000 pounds per square inch, tending to cause creep. 
Is this enough to overcome the strength of the rock, which opposes 
creep? Even the lesser of these figures is equai to the crushing 
strength of some of the weaker rocks, and is a notable percentage of 
the crushing strength of even the strongest. Under less pressure 


FORMATIONS AND PHYSICAL HISTORY  — 351 


than this, rock in some mines is observed to creep. It is not im- 
probable, therefore, that such a pressure, constantly exerted for 
a prolonged period, might cause some spreading of the great con- 
tinental platforms, and hence (1) some lowering of their surfaces, 
(2) some submergence about their borders, and (3) at the same 
time some rise of the sea-level. Many phenomena which cannot be 
cited here seem to lend support to this hypothesis of lateral creep,! 
but its efficiency is not determined. 


Sedimentation. in the Cambrian Period 


Sedimentation in the Cambrian period appears to have followed 
the general laws that govern deposition in periods of comparative 
freedom from great deforming movements. Most of the known 
sediments were deposited in the sea, and their area may be regarded 
as a rough measure of the area of the Cambrian sea. Sedimentation 
was probably faster in the early stages of the period when the land- 
area was largest and highest, and slower in the later stages after 
the land had been lowered and narrowed. Sedimentation was 
probably greatest near the land. 

Sources and kinds of sediments. As in other periods, the land- 
derived sediments came from all formations exposed to erosion. 
The sediments along the immediate borders of the land were doubt- 
less different from those farther out, and even along shore probably 
there were variations, because of differences (1) in the sources of 
the sediments, and (2) in wave, river, and current action. 

The Cambrian system includes all common phases of sedimen- 
tary rocks. There are conglomerates, presumably laid down near 
the shores of the time; sandstones, the sand of which was deposited 
in shallow water; shales, representing the mud deposits in quiet 
water; and beds of limestone representing, for the most part, the 
accumulations of shells, etc., where sediments from the land were 
not abundant. 

Geographic variations. The distribution of these various sorts 
of sedimentary rocks shows that various kinds of detrital beds were 
accumulating in different places at the same time, and at the same 
place at different times. Not only this, but they were accumulated 
at very different rates, as the great variations in thickness show. 

The fact that the Upper Cambrian in the northern interior of 
the United States is mostly of sandstone, and that this sandstone is 

1 Chamberlin and Salisbury, Earth History, Vol. II. 


352 '—- CAMBRIAN PERIOD 


widespread, indicates that the water was so shallow that the waves 
were competent to roll sand long distances. Furthermore, the 
structure of the beds, with their cross-bedding (Fig. 199), ripple- 
marks, etc., shows that the whole of the thick series from bottom to 
top was deposited in shallow water, and therefore on a surface which 
was depressed gradually, relative to sea-level, as the sand accumu- 
lated. The limestone (chiefly dolomite) in the Upper Cambrian of 
the southern and southeastern interior, points to clear seas, but per- 
haps not to deep ones. The adjacent lands were perhaps too low 
to yield abundant sediment. Limestone is also an important part 
of the Middle and Upper Cambrian of the west, though clastic rocks 
predominate in the Lower Cambrian. Where the Upper Cambrian is 
limestone, it is, as a rule, not sharply differentiated from the over- 
lying Ordovician. 
Outcrops of Cambrian 

The Cambrian formations were once as widespread as the Cam- 
brian seas themselves, but they are not now present over all the 
area they once covered. ‘The areas where they are exposed are not 
to be confused with the areas where they actually exist. Cambrian 
formations are exposed, for example, in Wisconsin, Missouri, and 
Texas; but the strata of Missouri are doubtless continuous, beneath 
younger formations, with those of Texas, on the one hand, and with 
those of Wisconsin, on the other (Fig. 310). 

Position of outcrops. The map (Fig. 310) showing the areas 
where the Cambrian system is now exposed reveals several points 
of significance: (1) Many of the outcrops are in association with 
outcrops of the Archean and Proterozoic systems (Fig. 295). In 
places, the exposed Cambrian lies along one border of the exposed 
parts of these older systems, while in others it completely surrounds 
them. ‘This distribution is not peculiar to the Cambrian, but is 
characteristic of most formations as compared with those of greater 
age. (2) The exposed areas of Cambrian in the Appalachian Moun- 
tains occur in parallel or subparallel belts (Fig. 310). This is the 
result of (a) the folding to which the Cambrian and later strata of 
this region have been subject, and (6) the erosion which the folds 
have suffered. Fig. 314 will help to explain the repetition of out- 
crops. In this diagram, A represents pre-Cambrian strata, € repre- 
sents the Cambrian, and O, S, D, and C, the Ordovician, Silurian, 
Devonian, and Carboniferous systems, respectively. After the 
strata were folded, erosion cut the folds down. A fold involving 


FORMATIONS AND PHYSICAL HISTORY 353 


Cambrian beds, if truncated below the level of the bottom of these 
beds at their highest point, exposes two belts of Cambrian strata, 
one on either side of a pre-Cambrian axis, as represented in the 





AVAUAR unt ere ae 





Fig. 312. Diagram illustrating the relation of Cambrian formations, A, B, and 
C, to older rocks. The diagram suggests that the Cambrian formations have been 
eroded back from their original margins, A’, B’, and C’. 





Fig. 313. Diagram illustrating the general relations of Cambrian beds in the 
interior. ‘The Cambrian, €, is represented as appearing at the extremes of the 
diagram, and as dipping below younger beds between. 


left-hand part of the figure. If the truncation is at a level below 
the top and above the bottom of the Cambrian (right-hand side of 





Fig. 314. Diagram showing the positions of outcrops determined by folds. 


Fig. 314), the strata of that system are exposed in a single belt along 
the axis of the fold. (3) In some places, Cambrian outcrops are 
surrounded by older 





formations. In such Pee eee 1a ae os 
bs sap nea gs® ay 
cases the Cambrian t cess Bel aare aN 
oo ae TS eed 
outcrops presuma- , Bi eve eee Vm Nad Tee he, tN 


bly represent rem- 

nants which have Fig. 315. Figure to illustrate isolated occurrences of 
escaped _ erosion. Cambrian surrounded by older formations. 

They may occupy depressions in the surface of pre-Cambrian 
formations, or may constitute hills (Fig. 315). 


354 CAMBRIAN PERIOD 


Width of outcrops. The widest outcrops of the Cambrian 
(Fig. 310) are in Wisconsin; yet there the Upper Cambrian only is 
present, with a thickness of less than 1,000 feet, while in the Appala- 
chian Mountains, where the system has anaggregate thickness of sev- 
eral thousand feet, it appears at the surface in narrow belts; that is, 
the outcrops are narrow in the east where the system is thick, and 
wide in the interior where it is thin. The explanation of this appar- 
ent anomaly is found in the attitude of the strata. In Wisconsin 
they are nearly horizontal, while in the mountain regions, both east 
and west, they are tilted at high angles. Where strata are vertical, 
the width of their 
outcrop on a hori- 
zontal surface is 
about the same as 
S&S the thickness of 

Fig. 316. Diagram illustrating the influence of dip the beds (€, right- 
on the width of outcrop. The Cambrian beds, €, to the hand side of Fig. 


left have a much wider outcrop than the Cambrian beds 316); where they 
to the right, though the thickness is the same. : 
are nearly hori- 


zontal, (€, left-hand side of Fig.) the width of outcrop on a hori- 
zontal surface is much greater. It is not to be inferred, however, 


that horizontal strata always have a wide outcrop. The width of 
outcrop is also influenced by topography, as shown in Fig. 317. 














Fig. 317. Diagram illustrating the effect of topography on width of outcrop. 


Here the horizontal stratum between B and C has about the same 
thickness as € of Fig. 316, but its outcrop is narrow. In general, 
the width of outcrop, so far as determined by topography, depends 
on the angle between the bedding-planes and the surface where the 
formation outcrops. The width of the outcrop decreases as this 
angle increases. 


Changes in Sediments Since Deposition 


The sediments of the Cambrian system have undergone change 
since their deposition. In most regions they have been compacted 
and cemented into solid rock. Over great areas in the interior 


FORMATIONS AND PHYSICAL HISTORY 356 


(Figs. 312 and 313) the strata still remain nearly horizontal, while 
in some other regions they have been tilted, folded, and faulted 












Bs pete " 
b frase 
Fok S eek 


SrNINe Cee 






Fig. 318. A section in the Menominee region of northern Michigan, showing 
the Potsdam sandstone, €s, in unconformity with older formations. (Van Hise, 
U. S. Geol. Surv.) 





Fig. 319. Section showing relations of the Cambrian in the Appalachian Moun- 
tains. ‘The strata are folded and faulted. -€, Cambrian; O, Ordovician; S, Silurian. 
Length, 13 miles. (Hayes, Cleveland [Tenn.] folio, U.S. Geol. Surv. Ordovician 
and Silurian not separated in the original.) 


(Fig. 319). Where close folding has taken place, the rocks have 
been more or less metamorphosed. In extreme cases the sandstones 











Fig. 320. Section showing the relations of Cambrian and other formations at 
a point north of Leadville, Colorado. -® gn, Archean; €s, Cambrian; Cmr, Carbon- 
iferous; Jw, Jurassic; /p and gp, igneous rocks. (Emmons, U. S. Geol. Surv.) 


have been converted into quartz schists, the shales into slates and 
schists, and the limestones into marble. 


Close of the Period 


No physical changes of great importance seem to have marked 
the close of the Cambrian period in America. Nowhere in our 
continent, so far as now known, were mountains made at this time, 
and nowhere were great areas of sea-bottom converted into land, 
though local unconformities between this system and the next record 
local changes in the sites of deposition. 


356 CAMBRIAN PERIOD 


The Cambrian in Other Continents 


Europe.! In Europe, as in North America, widespread defor- 
mation before the beginning of the Cambrian converted large areas - 
of the present continent into land, and there is evidence that these 
lands, like those of America, were subjected to protracted erosion 
before the deposition of the Cambrian system. 

The Cambrian system of Europe, like that of America, is largely 
clastic. Ripple-marks, cross-bedding, and sun-cracks are common, 
showing that a large part of the Cambrian sediments were laid down 
in shallow water, or on land. 

In Wales (Cambria), the country from which the system got its 
name, and in Brittany, the system is very thick. In Scandinavia 
and western Russia, on the other hand, it is thin, locally no more 
than 400 feet. These differences probably mean that sediments 
were being deposited in some places many times as rapidly as in 
others. The Middle Cambrian of Eu- 
rope is more widespread than the Lower 
or Upper, showing that changes in the 
relation of sea and land were in progress 
during the Cambrian period, shifting 
the areas of erosion and sedimentation. 

The Cambrian of western Europe 
has been much folded, but in central 
and eastern Europe, the strata are 
essentially horizontal. Beds of clay 
which are still plastic, and beds of sand 
which are still uncemented, are known 
in the undeformed part of the system. 
Geographic changes of great importance 
seem not to have marked the close of 





Fig. 321. Glaciated stone é ’ 
from the glacial beds at the the Cambrian, in Europe. 


base of the Cambrian in China. Other countries. Cambrian rocks 


(Willis, Carnegie Institution.) — 4¢cyr in various parts of Siberia, China. 


India, Australia, and Tasmania, and in the northwestern part of 
Argentina, but their distribution outside of North America and 
Europe is but poorly known. 

Glacial formations. (1) In northern Norway, Lat. 70° 8’ W., 


1 The best summary, in English, of the Cambrian of Europe, is found in Geikie’s 
Textbook of Geology, 4th ed., Vol. II. 


FORMATIONS AND PHYSICAL HISTORY 357 


there is a bowlder-bearing formation (the Gaisa beds) resting on a 
glaciated surface of crystalline rock. The Gaisa beds have been 
thought to belong to the oldest part of the Cambrian system, or to 
antedate it. (2) Recent exploration in China! has made known 
a thick formation (170 feet) of bowlder-bearing rock of glacial 
origin, containing many striated bowlders of diverse sorts of rock 
(Fig. 321) on the Yangtse River, in latitude 30°. This formation 
lies at the base of the Paleozoic, beneath the beds that carry 





Fig. 322. A glaciated bowlder from the Cambrian till of Petersburg, South 
Australia. (Howchin.) . 

Cambrian trilobites. Glacial formations of early Cambrian age 
have been found in Australia, and perhaps in South Africa.’ 

The profound climatic significance of these glacial formations 
is obvious. The testimony of Cambrian fossils, on the other hand, 
implies nearly uniform climatic conditions throughout all regions 
where fossils have been found, and the wide spread of the sea during 
the later part of the period would seem to point to oceanic, rather 
than continental, climates at that time. 


Duration of Cambrian Period 


There is no reliable estimate of the duration of the Cambrian 
period. The destruction and removal to the sea of such large 
volumes of rock as are represented by the sediments of the system 


1 Willis, Researches in China, Vol. II. 
2 David, Report of International Geological Congress at Mexico, 1907; and 
Howchin Quar. Jour. Geol. Soc., Vol. LXIV, p. 234, 1908, and Jour. of Geol., Vol. 


XX, pp. 193-8. 


358 CAMBRIAN PERIOD 


required a very long period of time; but since there is no standard 
rate at which any sort of sediment accumulates, this long period 
cannot be reduced to years. It has been estimated that limestone 
sometimes forms at some such rate as one foot per century. In 
some parts of the West there are 6,000 feet of limestone, besides 
thick bodies of fragmental rock. At the above rate of accumula- 
tion, 6,000 feet of limestone would call for a period of 600,000 
years, and if time be allowed for the other formations of the same 
region, this period would be lengthened greatly. It should be 
remembered, however, that while one foot per century is a rate 
at which limestone may accumulate, it does not follow that it is 
the rate at which Cambrian limestone was formed. 

Many estimates of geological time, based on various data, have 
been attempted.! These estimates, so far as applied to the Cam- 
brian, generally assign to that period a duration of 1,000,000 to 
3,000,000 years. It should be distinctly borne in mind, however, 
that the chief value of these figures is to give emphasis to the fact 
that the period was one of great duration. For aught that is now 
known, the largest of these figures might be multiplied by 2 or 
even by some larger number. 


LIFE OF THE CAMBRIAN 


Perhaps no single event in the history of the earth possesses 
greater interest than the first appearance of life; but the date of 
its beginning is not known. ‘There is good evidence that life existed 
before the close of the Archeozoic era, and under the accretion 
hypothesis, it is not improbable that its beginning antedated, by a 
long period, the oldest accessible Archean formations. If so, it 
is quite beyond hope that the earliest forms of life will ever be 
known from fossils. The known fossils from the Proterozoic rocks 
give but a very inadequate conception of life before the Cambrian. 
But in the Cambrian system there is, for the first time, a reasonably 
adequate record of animal life. 

Animal fossils. Every great division of the animal kingdom, 
except the vertebrate, was representated in Cambrian times, and 
though no vertebrate remains have yet been found, it would be rash 
to assume that no vertebrates lived. All the known fossils appear 
to be of marine species. Of land animals there are no traces, but 
this does not prove that they did not exist. 

1 For a general discussion of this topic, see Williams’ Geological Biology, Chap. II. 


LIFE 350 


-Trilobites were easily the most distinguished forms of Cambrian 
life. They were not only the highest in organization, but the most 
characteristic of the period. Their successive genera best distin- 
guish its successive stages, and their distribution is a chief means of 
correlating the formations of different regions. Figs. 311 and 323 
show their three longitudinal lobes, whence their name. Trilobites 
were kin to the modern crab and crayfish, representatives of the 
great group Arthropoda (p. 686). They have long been extinct, 
but the modern horse-shoe crab has some likeness to them. 

Trilobites were well advanced in the scale of development, 
possessing nearly all the anatomical systems and _ physiological 
functions of modern crustaceans. Perhaps their compound eyes 
are the best index of their development. In this and succeeding 
periods, the number of eyelets in trilobites’ eyes ranged from a 
score to several thousands. Some of them, however, had no eyes, 
while others possessed abortive rudiments, implying that their 
ancestors had possessed them. The acquisition and abortion of so 
important an organ seem to indicate change in the conditions of 
life. This may mean no more than migration to deep dark waters, 
or the habit of burrowing in the mud, where eyes were useless. The 
eyes of some were raised slightly on crescentic lobes, with the con- 
vex face outwards (a and c, Fig. 323). In later epochs, these cres- 









ae WAbspCE i 
COLL LLLP 


ff AN) Wey 


© SVE Ne 
f 
t 





Fig. 323. CAMBRIAN CRUSTACEA: a, Holmia (Olenellus) bréggeri Walcott, a 
characteristic trilobite of the Lower Cambrian; b, Olenoides curticei Walcott, a 
Middle Cambrian trilobite; c, Ptychoparia kingi Meek, a Middle Cambrian trilobite; 
d, Agnostus interstrictus White, a Middle Cambrian trilobite; e, Aristozowe rotundata 
Walcott a Cambrian phyllocarid; f, Leperditia dermatoides Walcott, a Cambrian 
ostracode. 


360 CAMBRIAN PERIOD 


cents became more and more curved, extending the sweep of vision 
fore and aft, to the animal’s obvious advantage. 

The upper surface of the body was ornamented variously, and 
the ornamentation varied as time went on, increasing, in general, 
until after the climax of the trilobites had been passed. ‘Trilobites 
possessed a row of slender articulated legs on either side, and deli- 
cate filaments which served the. function of respiratory organs. 
The nature of the legs indicates that trilobites both walked and 
swam. They possessed antennze which doubtless served as organs 
of touch, and they moulted the shell at successive stages of growth, 
like modern crabs. Omitting further details, it is to be observed 
that, at this early day, a highly complex, well-differentiated organi- 
zation had been acquired, possessing nearly all the organs and 
functions of arthropods of the present day. 

Brachiopods (molluscoidea, p. 686 and Fig. 324) were second in 
geological importance to trilobites; but unlike trilobites, brachiopods 
still live. They are conspicuous representatives of stability and 
persistence. Though the species and most of the genera have 





g h 

Fig. 324. CAMBRIAN Bracutopops: a and b, Acrotreta gemma Billings, a 
brachiopod ranging from the Lower to the Upper Cambrian, summit and side 
views of the ventral valve; c, Billingsella transversa Walcott, a pedicle or ventral 
valve of a hinged brachiopod of the Lower Cambrian; d and e, Lingulepis pinni- 
formis Owen, views of the two valves; f and g, Kutorgina cingulata Billings, side and 
dorsal or brachial views, a Lower Cambrian species; /, Billingsella coloradoensis 
(Shum.), an Upper Cambrian species. . 





changed, the class as a whole has been but slightly modified since 
the Cambrian period. The brachiopod shell is bivalve. The two 
valves are unlike, but each is bilaterally symmetrical (Fig. 324). 


LIFE 361 


Mollusks (p. 686) were well represented, Cephalo pods (chambered 
shells), the highest class of mollusks and are found in the upper- 
most beds of the Cambrian. As they were even then highly devel- 
oped, there is little doubt that the class had passed through a long 
history before the end of the period. Pelecypods (bivalves, oysters, 
clams, etc., 0, Fig. 325) lived throughout the period, though their 





Fig. 325. CAMBRIAN Mo.tusks: a, Hyolithes americanus Billings, a Lower 
Cambrian pteropod; 6, Fordilla troyensis Barrande, a Lower Cambrian pelecypod; 
c, Stenotheca rugosa Hall, a capulid gastropod of the Lower Cambrian; d, Trocus 
saratogensis Walcott, a gastropod with well-developed spire; e, Platyceras primevum 
Billings, a Lower Cambrian gastropod; f, Ophileta primordalis Winchell, an Upper 
Cambrian gastropod. 


fossils are not abundant. Like brachiopods, pelecypods are bi- 
valves, but unlike the brachiopods, the valves are not bilaterally 
symmetrical. Gastropods (univalves, c, d, e, Fig. 325) are rather 





Fig. 326. CAMBRIAN VERMES: borings and trails. a, a surface of sandstone 
showing annelid borings, with mounds of sand heaped about their mouths and with 
trails leading away from some of them, 


362 CAMBRIAN PERIOD 


plentiful throughout the system. The early forms are chiefly of 
the low conical type, while more amply coiled and spiral forms be- 
came common later. Some of them resemble modern gastropods 
closely. 

Sea worms (Vermes, p. 686) left evidence of their abundance by 
borings, tracks, etc. (Fig. 326). A few cystoids, the forerunners of 
the beautiful crinoids (stone lilies), represented the echinoderms. 

Celenterates were represented by graptolites, meduse and 
polyps (corals). The eccentric freaks of fossilization are nowhere 
better illustrated than here. Relics of graptolites, among the most 
delicate of animal forms, and of medus@ (jelly-fish), among the soft- 
est of animals, were preserved, while some stronger types left scant 
record of themselves. Graptolites, now extinct, were slender, plume- 





Fig. 327. CAMBRIAN C@LENTERATA: supposed corals, meduse, and grapto- 
lites. a and b, Archewocyathus rensselericus Ford, a problematic fossil referred by 
some paleontologists to sponges, and by others to corals; c and d, Brooksella alternata 
Walcott, supposed casts of the gastric cavities of meduse; c, a supposed exumbrella 
in which the interumbrella lobes are a prominent feature; d, a view of a supposed 
umbrella with six lobes and a depression over the central stomach; e, een 
(?) cambrensis Walcott, the hydrosoma of a graptolite. 


like organisms (e, Fig. 327), consisting of a series of hard cells, in 
which the individual zodids lived, attached to a common Bender 
axis. The whole colony appears to have floated free in the sea. 
The secret of their preservation probably lies in the fact that, being 
floating forms, they settled in quiet waters off-shore, where fine silts 
accumulated, and where the conditions were favorable for burial 
without destruction. The most singular case of fossilization is 
the preservation of traces of jelly-fish, or at least of what are so 
identified (Fig. 327, c and d) in the Lower Cambrian. Obscure 
fossils of corals are found (Fig. 327, a and 0), the forms of which re- 
semble sponges so much that they long were regarded as such. 
Corals seem to have been more abundant in some other parts of the 
world than in North America. | 


LIFE 363 


Sponges lived throughout the period. It is probable that many 
protozoans existed, but only a few forms have been identified. 

Implied life. The existence of so much animal life implies 
much vegetable life to supply the necessary food. Furthermore, 
various characteristics of the fossils suggest the presence of animals 
not known from fossils. A large percentage of the known Cambrian 
animals were provided with shells, tests, plates, or other forms of 
hard coverings. In the main, these appear to have been protective 
devices, and imply enemies or rivals against which protection was 
needed. Perhaps the most significant feature of the protective 
devices is that they are of the same types as those possessed by 
similar animals of later times. If there had been a radical change 
in the character of their enemies or rivals, we might expect some 
notable change in the defensive devices. It is a natural inference, 
therefore, that the conflicts of life in the Cambrian seas were similar 
to those of the present. ‘The inference may be pushed further, and 
the deduction drawn that the conflicts which led to the evolution of 
the defensive devices were much like those throughout the period of 
their retention. 

Stage of evolution represented. What stage of advancement 
in the development of life had been attained by the beginning 
of the Cambrian period? Do the fossils of the system indicate 
that the life of the period was primitive, or do they imply that it 
had advanced far beyond primitive forms? For comparison it 
may be assumed that the first forms of life were as simple as the 
simplest existing forms. If the plants and animals that consist of 
a single cell are taken to represent primitive forms, how far had 
the Cambrian life advanced beyond them? 

In the early stages of their development, animals pass through a 
succession of changes in which their structure resembles that which 
their ancestors had in their maturity; in other words, the individual 
history of any animal is an epitome of the history of its ancestors. 
Now the Cambrian trilobites are known to have passed through a 
series of remarkable changes after the individuals had developed 
far enough to be fossilized, and it is inferred they passed through 
other stages previously. There is, therefore, specific ground for 
believing that they had had a long line of ancestors. 

On the anatomical and physiological side, it is clear that nearly 
or quite all the fundamental organs had been developed. ‘There 
were skeletal systems of several forms, muscular systems, nervous 


364 CAMBRIAN PERIOD 


systems of high development, as implied by eyes and other sense- 
organs, devices for capturing and ingesting food, organs of digestion, 
secretion, excretion, and respiration. ‘The Cambrian animals had 
acquired the various habits of life possessed by existing animals of 
their kind, as well as the various modes of preserving their lives. 

The question may be approached in another way. The studies 
of recent decades have convinced investigators that later forms of 
life were derived from earlier ones by processes of evolution. The 
exact methods of evolution are not altogether understood, but the 
fact of evolution is not now regarded as an open question. As the 
various forms developed and diverged from a common ancestral 
stock, many of the intermediate forms disappeared, and the forms 
which persisted became widely separated. By continued diver- 
gence, with the loss of intermediate types, a discontinuous series of 
forms was developed, and those which lived on became more and 
more unlike. The process was not unlike the evolution of a tree- 
top, in which the dying out of most of the interior branches leaves a 
few great limbs which bear the more numerous and more recent 
branches, while these in turn bear the uppermost and outermost 
twigs which represent the living phase. In some such way, it is 
thought that the existing divergence of organisms into kingdoms, 
branches, classes, orders, families, genera, species, and varieties 
came to be established. 

If it is assumed that the whole system of living things was derived 
from a common primitive form, or from a few primitive forms, a 
comparison of the primitive state with the degree to which life had 
advanced in the Cambrian period will give some impression of the 
amount of pre-Cambrian evolution. If to this be added a compari- 
son between the Cambrian life and that of today, an estimate of the 
relative amount of evolution before and since the Cambrian may 
be made. 

It is to be noted that not only were all the animal sub-king- 
doms, save perhaps the vertebrate, present, but that, in many of 
them, the species had come to have nearly the aspect of living 
forms. The initiation and divergence of the structures and types that 
preceded the Cambrian stage mean much more in the way of evolution 
than all the evolution of later times. These considerations lead to 
the conclusion that life must have been in existence a very long time 
prior to the Cambrian period. 

The succession of faunas. Under the doctrine of evolution, it 


LIFE 365 


is presumed that the life of every past stage has grown out of that 
which immediately preceded it, and that it has merged into that 
which immediately followed it. It is usually assumed that if no 
* exceptional influences came in, there was a continuous series of slow 
changes without sharp lines of demarkation. If this conception 
were realized in fact, it would be less appropriate to speak of a suc- 
cession of faunas than of one continuous ever-changing fauna. It 
is not yet demonstrated, however, that evolution proceeded solely 
by very slight changes coming in from generation to generation. It 
may have proceeded by distinct and abrupt changes;! or at any 
rate new species may have arisen abruptly, so far as now known. 
Irrespective of any other specific hypothesis, it is to be noted that 
the geological record, as now known, does not show complete grada- 
tions from one species to another. In some cases there is something 
of a graded series, but the steps of the gradation are not sufficiently 
close and definite to decide between evolution by an infinite number 
of small changes, and a smaller number of greater changes. 

If we turn from species to faunas, a more general point of view 
must be taken. Observation shows that in some cases one fauna 
grades into the succeeding one, while in other cases the change 
appears to be abrupt. If the progress of life the world over could 
be studied as a unit, it would probably appear that there was a 
nearly perfect gradation of the life of one stage into that of the next. 
This gradation probably was more rapid at some times than at others, 
and it is quite certain that some forms changed more rapidly than 
others. But when we limit our study to the succession of faunas on 
any one continent, or to any one province, it is evident that the 
progress of evolution in the region studied was interrupted by physi- 
cal changes which affected the depth, temperature, or clarity of the 
water, and the nature of the bottom, and that these changes brought 
about variations in the character and distribution of life. There 
seem to have been rather definite times of notable change, between 
which faunas changed but slowly. Where the faunal change in a 
conformable series is abrupt, and there is no evidence of a gap in the 
record, the explanation is usually sought in the immigration of a new 
fauna from some other region. 

In the study of faunal progress, therefore, there is occasion 


1 DeVries. Die Mutationstheorie, 1903. See also Bateson’s Material for 
the Study of Variation, 1894; W. B. Scott, On Variations and Mutations, Am, 
Jour. Sci., 1894. p. 355; and discussions of Mendel’s theory. 


366 CAMBRIAN PERIOD 


to recognize (1) rather abrupt changes brought about by over- 
whelming invasions; (2) less abrupt changes brought about by 
the more gradual ingress of outside species, and the gradual com- 
mingling of immigrants with resident species; (3) very gradual’ 
changes due to the slow evolution of resident species when not much 
affected by immigration or by physical changes; and (4) rapid 
evolution due to profound changes in the physical conditions or to’ 
other agencies less well understood. 

The abrupt appearance of the Cambrian fauna. The apparent 
suddenness of the appearance of the Cambrian fauna is unexplained. 
In a general way, it may be said that older formations have been 
metamorphosed, and that this destroyed most of their fossils; but 
this suggestion is not altogether adequate, for some of the older 
formations are not greatly changed, and some younger metamorphic 
rocks carry fossils. It is also true that some younger formations 
which seem well suited to receiving and retaining organic impres- 
sions are without them. Geologists are inclined to refer the scanti- 
ness of pre-Cambrian fossils, and hence the apparent abruptness of 
the introduction of the Cambrian fauna, to unfavorable conditions 
for fossilization in pre-Cambrian time, combined with subsequent 
changes in the rock. This makes the abruptness a matter of rec- 
ord, rather than of fact. 

Map work. Suggestions for work with geologic folios are found in Laboratory 


Exercises in Structural and Historical Geology, SALISBURY AND TROWBRIDGE, 
Exercise VIII. 


CHAPTER XVI 
THE ORDOVICIAN (LOWER SILURIAN) PERIOD ! 


FORMATIONS AND PHYSICAL HISTORY 


The general conformity ? between the Cambrian and Ordovician 
systems shows that no great change took place in the relations of 
land and water in North America at the close of the Cambrian period. 
At the opening of the Ordovician, therefore, an epicontinental sea 
stood over much of the continent. 


Sedimentation During the Ordovician Period 


While the principles of sedimentation during this period were 
- the same as during the Cambrian, the conditions, so far as our 
continent is concerned, were somewhat different, chiefly because 
the smaller areas of land yielded less sediment. During much of 
the period the deposition of land-derived detritus was confined to 
littoral tracts. Since the land areas were of various sizes, of various 
sorts of rock, and presumably of various heights, conditions existed 
for the deposition of all sorts of clastic sediments about their borders, 
and for their deposition at very different rates. Sedimentation was 
doubtless more rapid near the larger and higher lands than about 
the smaller and lower ones, and more rapid on that side of any land 
towards which the larger part of its drainage flowed. Where clastic 
sediments failed, the shells and other secretions of marine animals 
and plants were accumulating, making limestone. 

The known formations of the Ordovician period are in keeping 
with these general principles. Adjacent to the broad, shallow 


1 Recently it has been proposed to recognize a system of rocks, the Ozarkian, 
between the Cambrian and the Ordovician, the Ozarkian would include the lower 
part of the Ordovician (Beekmantown formation and its equivalents), and the 
upper formations of certain regions commonly referred to the Cambrian. Ulrich, 
Bull. Geol. Soc. Am., Vol. XXII. 

* There are local unconformities between these systems, as in some parts of 
New York, and the evidence is increasing that they are more wide-spread than 
formerly was supposed. 


307 


368 ORDOVICIAN PERIOD 


arm of the ocean which covered the larger part of the Mississippi 
basin (Fig. 310) there appear to have been no sources of abundant 
sediments during most of the period. Along the western base of - 
Appalachia, clastic materials were being deposited. Alternating 
beds of coarse and fine sediment indicate either (1) that the adjoin- 
ing land was higher at some times than at others, or (2) that the 
climatic conditions or (3) the vegetal covering changed, or (4) that 
waves and currents varied in their effectiveness. 

Conditions for the formation of limestone prevailed widely in 
the epicontinental sea. Plants and animals secreting calcium car- 
bonate may have been no more abundant far from land than near 
it, but away from shore their shells, etc., were more abundant 
relative to the sediments derived from the land. 

The development of the Ordovician system meant the destruc- 
tion of an equivalent body of older rock. The material which 
entered into the new system came from all preceding formations 
so situated as to be exposed to erosion. Even the limestones of the 
system had their ultimate source in older formations, for the mineral 
matter extracted from the sea to make the shells had been dissolved 
from older formations during their decay, and brought to the sea in 
solution, largely by the same streams which carried the clastic sedi- 
ments. 

Sections of the Ordovician. The Ordovician system of North 
America was first studied carefully in New York, and the section of 
that State is, in some measure, the standard to which others are 
referred. In New York the system is divided as follows: 


Richmond beds! (in Ohio and Indiana) 
Lorraine beds 

Utica shales 

Trenton limestone 


Upper Ordovician 
(or Cincinnatian) 


Ordovician ; Middle Ordovician Black River mnestaee 


(or Mohawkian) Lowville limestone 
Lower Ordovician Chazy limestone 
(or Canadian) Beekmantown limestone (Calciferous) 


The classification of New York is not applicable in detail in other 
parts of the continent. In Wisconsin, Iowa, and Minnesota, for 
example, the formations commonly recognized, numbered in the 


1 Question has been raised as to the propriety of including the Richmond beds 
in the Ordovician. Hartnagle, N. Y. State Mus. Bull. 107, 1907. In Illinois, 
beds of Richmond age are unconformable on the older Ordovician. Weller, Jour. 
of Geol., Vol. XV, p. 519; and Savage, Am. Jour. Geol., Vol. 125, p. 431, 1908, » 


FORMATIONS AND PHYSICAL HISTORY 369 


order of age, are shown below, but it cannot be affirmed that any 
one of them is the exact equivalent of any one in New York. 
Upper Ordovician 5. Hudson River ! (Maquoketa) shale 


Poatie Ordovician } 4 eae ene 
3. Trenton limestone 


( 2. St. Peter sandstone 


Lower Ordovician ; ; 
1. Lower Magnesian limestone 


In the mountains of Tennessee, a series of limestone or dolomite 
beds (Knox, Chickamauga, etc.), is followed by a series of clastic 
beds (Sevier shale, Bays sandstone, etc.).2 The exact relations of 
these formations to those of New York and to those of the upper 
Mississippi basin are undetermined. The section of Tennessee 
does not correspond in detail with that of other parts of the Appala- 
chian belt. 

In the Great Plains, the Ordovician system appears at the sur- 
face but rarely, though it probably underlies the younger formations. 





Fig. 328. Trenton Falls, Trenton, N. Y. The locality whence the Trenton 
formation derived its name. (Darton, U. S. Geol. Surv.) 


1 Tt is now held by some that a portion, if not all, of the Hudson River (Maquo- 
keta) shale of the Mississippi basin is the equivalent of the Richmond beds farther 
east. 

2The subdivisions mentioned here are those of the Maynardsville, Tenn., 
folio, U. S. Geol. Surv. 


340 ORDOVICIAN PERIOD 


West of the Great Plains, the 
system is present generally, 
and the sections are somewhat 
simpler than in the interior 
or the east, limestone being a 
conspicuous part of the system 
here. 

General conditions in the 
eastern part of the continent. 
At no previous epoch was there 
anything like such widespread 
deposition of limestone within 
the limits of our continent, as 
in mid-Ordovician time, when 
limestone was forming from 
New England on the east, to 
Georgian Bay on the north- 
west, to Oklahoma and Texas 
on the southwest, and Alabama. 
on the south, as well as in much 
of the west. It is perhaps 
equally worthy of note that in 
the later part of the period, 
mud (now shale) was deposited 
over an almost equally exten- 
sive area. This may mean that 
the lands were so elevated as 
to allow the streams to carry 
more sediment to the sea, or 
that conditions favored the 
transportation of mud farther 
from shore than formerly, or 
both. All the Ordovician for- 


= Cambro-Ordovi- 
(Darton, Monterey 


€0 


eee 2), SD) ANN 2 
Sea) WG (a> > PEPE 
SO IMWMWhil? == AZZ PAWN 


43 


aS 


4 
2 
3 
4 


The last four formations are Ordovician, un- 


Sn, Niagara limestone (Silurian); Di, Hamilton limestone (Devonian, ) 


t a point in West Virginia. 
Length of section, 18 miles. 


Devonian. 


Siluro-Devonian; D= 


Length of section, about 135 miles. 


The section extends from the Archean area, in the north-central part of the state, to Lake Michigan, 
Silurian; SD 


in the vicinity of Milwaukee. AR, Archean; -€, Potsdam sandstone (Cambrian); Olm, Lower Magnesian Limestone; Osf, St. 


Peter sandstone; Of, Trenton and Galena limestone; Ofr, Hudson River shales. 


less, as recently suggested, p. 368, the last be Silurian. 


Section showing the relations of Ordovician and other beds a 


Fig. 329. Section of the formations in southern Wisconsin, showing the position of the beds and the relations of the several 





Los! 

o 

8 A 

a — 

_- fo) 

vo oO 
. — —/Y 
m Quy Oo 
oO ey nt . . . 
Ss x ¢,; mations of the interior and 
° Ppa a - “4 
g 5 35 the east bear within them- 

> ‘ 

Y a 3. selves evidence of shallow water 
O° ‘oO oO a) . . 
6 8 mow. OFgin. 
~~ to} 8) e =n ay 9 
r ir eae Igneous rocks of Ordovician 
8 = .-- age attain little importance in 
Nn . . 
> = ‘$= North America. Their general 


FORMATIONS AND PHYSICAL HISTORY 371 


absence is in harmony with the quiet which characterized the 
period. 


General Conditions and Relations of the Ordovician System 


Position of beds. As originally deposited, the Ordivician beds 
probably dipped away from the lands of the period. Over great 
areas in the interior, this original and simple plan of stratigraphy 
has been but little modified (Fig. 329). In other regions, deforma- 
tion of the strata has completely changed their original positions. 
Thus in the Appalachian Mountains (Fig. 330) and in some parts of 
Arkansas (Fig. 331), Oklahoma, oot 
and various mountains of the \ uA | fas 
west, the strata are folded and 
in some places faulted. 

Metamor phism. The sediments 
have undergone more or less al- 
teration since their deposition. In 






‘ 

A 
oO" 
\ 


: 
4 
or 
‘T—— 
We ALY 
a : 
\ H 
‘ 
\ % 





1} 
s 
bi) AA 
A 
‘ 
1 
a 


s 
oO 
AN 


\ 
\ 


' 
' 
i 

. X 
‘ ‘ . 
i 


\ \ \ 
some places the changes have ape iN : 
been slight, and in others great. rage + ee / a 

8 3 ¢ Ke Mi See wae we *e 
The larger part of the Ordovician yo Sn i “s 


"“ aseoe 


sands have been changed to sand- Fj oe ; 

g. 331. Section showing the 
stone, the larger part of the muds position and relations of the Ordovi- 
to shale, and most of the lime- cian beds in the mountains of Arkan- 
stone is still essentially non-meta- Pe ee le nee 
morphic. But where dynamic 
action has been great, and where the original position of the strata 
has been changed greatly, the changes in the rock have been 
greater.! Thus in the Taconic Mountains (southeastern New York 
and southwestern New England), the limestone has been changed 
to marble, the sandstone and quartzite to quartz schist, and the 
shale to slate and schist. 

Thickness. The rocks of all systems vary greatly in thickness, 
and the Ordovician system is no exception. In the Appalachian 
Mountains it is thousands of feet thick, while in the interior it is 
only hundreds. In Wisconsin and Iowa, the aggregate thickness 
is rarely more than 800 or goo feet. 

Outcrops. In the interior, where the system is relatively thin, 


1 See, for example, the New York City, Holyoke (Mass.-Conn.), and Hawley 
(Mass.) folios, U. S. Geol. Surv. Compare with folios of the Appalachian Moun- 
tains, the interior, and the western part of the United States. 


2 ORDOVICIAN PERIOD 


Y 


‘ 
Soe man, 


Fig. 332. Map showing the general condition of the North American continent 
in Mid-Ordovician (Trenton) time. The black portions represent areas where the 
Middle Ordovician beds appear at the surface. These areas so nearly correspond 
with the areas where the Ordovician system as a whole appears at the surface, that 
no serious error is involved if the black areas be interpreted as Ordovician. The 
various conventions of the map are the same as in Fig. 310, p. 347. 





FORMATIONS AND PHYSICAL HISTORY 373 


it appears at the surface in relatively wide belts or areas (Fig. 332), 
while in the eastern mountains, where it is thick, it appears at the 
surface in a succession of narrow and parallel belts (p. 372). The 
outcrops are largely adjacent to older rock. 


Close of the Ordovician Period 


The close of the period was marked by geographic changes of 
more importance than those at its beginning. The greatest change 
was the withdrawal of the epicontinental waters from a large part 
of North America, converting extensive stretches of shallow-sea 
bottom into land. The cause of this change may have been the 
sinking of the ocean bottoms and the drawing off of the epiconti- 
nental waters. The altitude of the new land must have been slight 
_ or its exposure brief, for it suffered little erosion before much of it 
was again submerged and covered by sediments of later age. It is 
indeed the widespread absence of the lower part of the Silurian 
system, apparent or real (p. 388), rather than a pronounced strati- 
graphic unconformity between it and the Ordovician, which indi- 
cates the extensive emergence of land in the interior at the close of 
the Ordovician period. 

Folding movements were limited. The most considerable were 
in the Taconic Mountains, where both the Cambrian and Ordovi- 
cian systems were thick. The date of the folding is known, because 
Silurian formations overlie the Upper Ordovician unconformably 
in this region. It is not to be inferred that all the mountain-making 
movements which have affected western New England occurred 
at this time. There had been folding earlier, in pre-Cambrian 
times, and there were movements later as will be noted. The 
principal deformation of the strata in the Appalachians and in 
Arkansas came much later. 

Between folding and the more gentle movements already noted 
there are all gradations. The ‘‘Cincinnati arch” is an example. 
This arch is a very low anticline with a general north-south course, 
extending through Cincinnati. The beginning of this arch may 
have been as early as mid-Ordovician. Another similar arch! may 
have come into existence at about the same time in Arkansas and 
Oklahoma. 

The crustal movements referred to above have been mentioned 


1 Branner, Am. Jour. Sci., Vol. IV, 1897, p. 357. This very suggestive article 
has bearings on many questions besides the Ouachita Uplift. 


374 ORDOVICIAN PERIOD 


as occurring at the close of the Ordovician. It would perhaps be 
more accurate to say that their beginning marks the beginning of 
the transition from the Ordovician period to the Silurian. The 
duration of the interval of transition was probably long. 


Economic Products 


In Ohio and eastern Indiana the Trenton formation yields much 
gas and oil.!. Both are commonly held to be products of the decay 
or distillation of organic matter included in the sediments when they 
were deposited. The oil is most abundant under low anticlines, 
where it occurs in the pores and openings of the rock, somewhat as 
ground-water does. 

The Galena and Trenton formations in Wisconsin ” and in the 
adjacent parts of Iowa and Illinois contain ores of lead and zinc, - 
mainly in the form of sulphides and carbonates. Lead ores are 
also found in the Ordovician (or Cambro-Ordovician) formations 
of southeastern Missouri,? and lead and zinc ores in the south- 
central part of the same state. In all these regions the ores occur 
(1) in cavities formed by solution, (2) as replacements of limestone, 
or (3) in crevices. In these positions, the ore was concentrated by 
ground-water. The metallic substances were doubtless derived 
from the limestone itself, which, at the time of its deposition, is 
thought to have contained trifling amounts of lead and zinc, perhaps 
extracted from sea-water by organisms. 

The Ordovician limestones of central Tennessee * locally yield 









































Fig. 333.. Shows modes of occurrence of the phosphates (the shaded surface 
parts of the limestone, ph) in central Tennessee. (Hayes and Ulrich, Columbia 
(Tenn.) folio, U. S. Geol. Surv.) 


1 Orton, 8th Ann. Rept., U. S. Geol. Surv.; Phinney, 11th Ann. Rept.; also the 
reports of the State Geol. Surv. of Ohio and Indiana. 

2 Chamberlin, Geol. of Wis., Vol. IV, 1879, pp. 365-568; Calvin and Bain, 
Iowa Geol. Surv., Vol. VI, and Grant, Bull. XIV, Wis. Geol. Surv., 1906. 

3 Winslow, Missouri Geol. Surv., Vols. VI and VII, and Buckley, Vol. IX. 

4Hayes. Columbia (Tenn.) folio, U. S. Geol. Surv. 


FORMATIONS AND PHYSICAL HISTORY a6 


calcium phosphate, valuable as a fertilizer. The workable deposits 
have resulted from the leaching out of the calcium carbonate from 
the phosphatic limestone, leaving the less soluble calcium phosphate 
concentrated at the surface (Fig. 333). The manganese ore of 
Arkansas had a similar origin. 


Foreign Ordovician 
The Ordovician formations appear at the surface in various 
parts of Europe, and they exist beneath younger formations over 
considerable areas where not seen. Fig. 334 represents the general 























































































































































































































Fig. 334. Diagram showing the relations of land and water in western Europe 
in the Ordovician period. The shaded parts represent areas of marine sedimenta- 
tion. (After DeLapparent.) 


geographic relations of land and water in Europe during this period. 
The submerged area represents in a general way the area where the 
Ordovician formations are present. In contrast with North Amer- 
ica, the Ordovician formations of Europe are largely fragmental. 
In the British Isles Ordovician strata are very thick (something like 


376 ORDOVICIAN PERIOD 


24,000 feet maximum).! Locally (Wales), nearly half the system is 
of igneous rock, including sheets of lava and beds of pyroclastic 
material. This is one of the most extensive, as well as one of the 
most ancient, volcanic tracts of Europe. From north England and 
Wales the system thins in all directions. 

The Ordovician of Europe is generally conformable on the 
Cambrian, but over considerable areas it is unconformable beneath 
the Silurian. 

In other continents the Ordovician strata have not, as a rule, 
been separated from the overlying Silurian, but they are known 
in South America, Australia and China. 


Duration and Climate 


The duration of the Ordovician is perhaps no better known than 
that of the Cambrian, but the period was probably somewhat 
shorter than its predecessor. 

Neither in Europe nor in America is there decisive evidence of 
distinct climatic zones. All that is known of the life would seem to 
indicate that the climate was much more uniform than now where 
the strata of the period are known. The fact that Ordovician 
rocks have been identified in the far north by fossils akin to those of 
low latitudes, indicates that the climatic conditions of North America 
and Europe must have been less diversified than now. This appar- 
ent lack of diversity of temperature through wide ranges of latitude 
is one of the unexplained problems of geology. Its solution is 
possibly to be found in a much higher average temperature of the 
ocean.” If the body of the ocean-water was relatively warm (in- 
stead of cold as now), it would have done much to counteract the 
effect of slight insolation in high latitudes during the cooler part of 
the year. 


LIFE 


From Cambrian to Ordovician, there was no pronounced break 
in the succession of life. The time from the beginning of the first 
to the close of the second of these periods appears to have been 
one long eon of progressive development and expansion of life. 
The fossil record of the Ordovician is fuller than that of the Cam- 
brian. ‘This is due partly to an increase in fossilizable forms, partly 


1 This measurement is doubtless subject to the strictures set forth on p. 355. 
2 Chamberlin and Salisbury, Earth History, Vol. III, pp. 437-445. 


LIFE 377 


to an increase in numbers of individuals, and partly to better con- 
ditions of preservation. 

The general aspect of life was cosmopolitan, though it was not 
che same everywhere. It varied with the physical evolution of the 
continent, and largely as the result of it. The variations assumed 
three general phases: (1) adaptation to the immediate physical 
environment, particularly the nature and depth of the sea-bottom; 
(2) modification by auto-evolution within areas isolated by barriers 
(provincial evolution); and (3) modification toward a universal type 
through intermigration (cosmopolitan development). 

(1) Rocky, sandy, muddy, and calcareous bottoms had their 
appropriate life, as did also tracts of shallow and deep water. The 
faunas adapted to these special conditions were not altogether un- 
like, for some animals, particularly free-swimmers, were indifferent 
to them. 

(2) Although the sea covered a large part of the continent, 
affording facilities for the migration and mingling of faunas, there is 
evidence of some separation into zodlogical provinces. ‘This was 
probably due partly (a) to barriers in the form of shoals, bars, and 
spits, (b) to ocean-currents with their attendant differences in tem- 
perature, and (c) to variations in the saltness of the waters. 

(3) Notwithstanding local and provincial modifications, the 
progress of Ordovician life in the American continent seems to have 
been, on the whole, in the direction of cosmopolitanism, especially 
in the shallow water faunas of the great interior of the continent. 
This was due, primarily, to the wide epicontinental seas, which 
permitted free migration. 

The Ordovician system contains an exceptionally large number 
of fossils of free-floating graptolites! (Fig. 343). Their remains are 
mingled with the fossils of shallow-water life, showing that they 
swam over the epicontinental seas freely. The Ordovician grapto- 
lites are nearly identical in Europe, North America, and Australia. 
The history of individual species was short, geologically speaking, 
and hence the succession of species marks the progress of events in 
all parts of the ocean. During the lifetime of the graptolites 
(limited to the late Cambrian, Ordovician and Silurian), a score of 
successive zones, each characterized by particular species, have been 


1Tt is not universally agreed that all graptolites were floating forms at all 
stages, but there seems to be little doubt that they usually were in their young 
stages at least. 


378 ORDOVICIAN PERIOD 


identified. One of these zones falls in the Cambrian, eight in the 
Ordovician, and eleven in the Silurian. If these be taken as chrono- 
logical bench-marks, the successive horizons of the different conti- 
nents may be correlated accurately,.and the progress of life in the 
various quarters of the globe referred to a common standard. 

Marine life. The known faunas of the Ordovician consist 
almost wholly of marine invertebrates, among which trilobites and 
brachiopods hold the leading places. Brachiopods are most numer- 
ous, trilobites highest in organization, and cephalopods most power- 
ful; but the foreshadowings of a new dynasty are at hand, for re- 
mains of fish are found in this system. 


cz 


\\\\ 


ea He ER 1B Ss 


ae = oy <j 
mee BARE 





Fig. 335. ORDOVICIAN TRILOBITES: 4, Jsotelus maximus Locke; 6, Ceraurus 
pleurexanthemus Green; c, Trinucleus ornatus Sternberg; d, Plerygometopus calli- 
cephalus (Hall); e, Préetus parviusculus Hall; f, Bumastus trentonensis (Emmons); 
g, Calymene callicephala Green. 


LIFE 37G 


Trilobites reached their climax in the Ordovician period, more 
than half of all the known genera being represented in the Ordovi- 
cian system. But few of them lived over from the Cambrian. In 
the next period the numbers fell off a full half, and the decline con- 
tinued until the tribe became extinct. The general aspect of the 
. trilobites at the high tide of their career is fairly illustrated in Fig. 
335. There was little or no increase in average size, as compared 
with their Cambrian forbears. Some individuals had a length of 
18 inches; but this size was equaled and even surpassed by some of 
their predecessors. 

Cephalopods. The largest, most powerful, and perhaps most 
predaceous forms of Ordovician life, seem to have developed into 


C O See) M P Pe T 








Fig. 336. The two upper curves represent the history of the trilobites according 
to genera, the full line indicating the total number of genera, and the dotted line 
the number of new genera introduced. ‘The two lower curves present the same data 
for the families of the trilobites. Data for families from Beecher in the Zittel- 
Eastman text-book of Paleontology, Vol. I. Data for genera, somewhat incom- 
plete, from Zittel’s “‘Handbuch der Paladontologie.”” -€, Cambrian; O, Ordovician; 
S, Silurian, etc., Pe, Permian, and 7, Trias. 


prominence suddenly. Unless the fishes, of which little is known, 
contested their supremacy, they were doubtless the undisputed 
masters of the sea. Their general aspect is seen in Fig. 337. The 
dominant type, as well as the most primitive one, had long, straight, 
gently tapering shells (Fig. 337, c and f) divided into chambers by 
plane partitions (sepia). Even in the Ordovician period there was 
a wide departure from this ideal simplicity. There were curved 


380 ORDOVICIAN PERIOD 


forms and coiled forms, some of which resemble the Nautilus of 
to-day (Fig. 337, e). Straight forms predominated, however, 
and the sutures (junctions of the septa with the outer shell) were 
simple, in marked contrast with some of those of later periods. 





Fig. 337.. ORDOVICIAN CEPHALOPODS: 4, Poterioceras apertum Whiteaves; }, 
Cyrtoceras neleus Hall; c, Orthoceras bilineatum Hall; d, Oncoceras pandion Hall; e, 
Trocholites ammonius Conrad; f, Orthoceras sociale Hall. 


In size, Ordovician cephalopods were probably never surpassed by 
representatives of their class. Some of the shells were 12 or 15 feet 
in length, and a foot (maximum) in diameter. From this great size 
they ranged down to or below the size of a pipe-stem. Unlike many 
mollusks, cephalopods were free swimmers. Gastropods, the kin of 
modern snails, were well represented in the early Ordovician fauna 
by diverse forms (Fig. 338). Few types of early Paleozoic life so 
closely resembled their modern relatives. Pelecypods (Fig. 338), 
the class to which clams and oysters belong, were subordinate to 
gastropods. Like their modern relatives, the Ordovician pelecy- 
pods seem to have been fond of muddy and sandy bottoms, for 





mM 
Fig. 338. OrpoviciAN GAsTRropops: @, Subulites reguiaris U. and S.; b, 
Maclurea logani Salter; c, Lophospira helicteres (Salter); d, Cyclomena bilix (Conrad); 
e, Schizolopha textilis Ulrich; f, Conularia trentonensis Hall; g, Hormotoma gracilis 
(Hall); 2, Eccyliomphalus triangulus Whitfield; 7, Helicotoma planulata Salter; 7, 
Cyrtolites ornatus Conrad; k, Raphistomina lapicida (Salter); 1, Protowarthia can- 
cellata (Hall); m, Bellerophon clausus Ulrich; n and 0, Archinacella cingulata Ulrich. 













2 


Fig. 339. ORDOVICIAN PELECYPops: a, Pterinea demissa (Conrad); 6, Bysso- 
nychia radiata (Hall); c, Vanuxemia dixonensis M. and W., interior of right valve, 
showing the hinge and muscular impressions; d and e, Ctenodonta nasuta (Hall); f, 
Lyrodesma cincinnatiensis Hall, interior of right valve, showing a primitive type of 
hinge; g,; Clenodonta recurva Ulrich; h, Ctenodonta pectunculoides Hall; 1, Rhytimya 


yadiata Ulrich, exterior of right valve. 





382 ORDOVICIAN PERIOD 


they are rather rare in the limestone beds of the early and middle 
Ordovician, and more abundant in the later shales. 

Brachiopods were still very abundant. Some were very similar 
to those of the Cambrian; but the higher, articulate forms (valves 
of the shell articulating) greatly outnumbered them. Among the 
articulate forms, the length of the hinge was increased, apparently 
affording a better means of resisting the attempts of enemies to 
reach them by sliding or rotating the valves past one another 
(i and , Fig. 340), while in others the margins of the valves were 





Fig. 340. ORDOVICIAN BrAcHiopops: a, Rafinesquina alternata (Emmons); b, 
Platystrophia lynx (Eich.); c, Hebertella sinuata (Hall); d, H. sinuata (Hall), interior 
view of the brachial valve, showing muscular impressions and hinge; e, Schizotreta 
ovalis H. and C., a pedicle valve; f, Trematis millepunctata Hall, pedicle view of a 
complete shell, showing the unmodified notch-like pedicle opening; g, Lingula 
rectilateralis Emmons; h-i, Strophomena subtenta Conrad, posterior view of a com- 
plete shell, showing the hinge-line, etc., and the exterior of the concave pedicle valve 
(i); 7, Crania lelia Hall, brachial views of four individuals attached to another shell; 
k, Schizocrania filosa (Hall), a brachial valve; 1, Leptena rhomboidalis Wilck, the 
pedicle valve; m, Orthis tricenaria Conrad, exterior of the brachial valve and the 
cardinal area of the pedicle valve; n, Rhynchotrema capax Conrad; 0, Dalmanella 
testudinaria (Dal.), brachial view; p, Plectambonites sericeus (Sow.), brachial view; 
q, Catazyga headi (Bill.), brachial view; r, Zygospira recurvirostris (Hall), interior of 
a brachial valve, showing the spiral brachidium in position. Compare Fig. 324. 


PLES 383 


notched so that the valves interlocked. In addition to these devices 
for preventing the opening of the shell, there was generally a thick- 
ening of the valves, and in many cases a ribbing of the exterior, 
giving strength without needless weight. These devices seem to 
imply that the enemies of the brachiopods had increased in effective- 
ness, but the abundance of the brachiopods implies that their ene- 
mies did not gain the mastery. 

Bryozoans (Fig. 341), kin to the brachiopods (p. 686) were 
very unlike them in external form, in habits, and in their hard 





Fig. 341. ORDOVICIAN Bryozoans: 4, Constellaria polystomella Whitfield; 2, 
Crepipora hemispherica Ulrich; c, Stromatopora delicatula (James); d, a part of c, 
enlarged; e, Callopora pulchella Ulrich, f, a part of e enlarged; g, Rhinidictya mu- 
tabilis Ulrich, h, a part of g enlarged. 


secretions. They lived in colonies, which secreted calcareous mate- 
rial. These secretions resemble coral so closely that they have 
been mistaken for it. In the middle and later portions of the 
period, the secretions of bryozoa contributed much of the limestone. 

Echinoderms (p. 686), represented now by such forms as star- 
fish and sea-urchins, were plentiful. The cystoids reached their 
climax before the close of the period; the crinoids became prominent, 


384 ORDOVICIAN PERIOD 


and the starfish and sea-urchin types had made their appearance. 
The cystoids (a, b and c, Fig. 342), with their irregular forms, were the 
most primitive, and gave place in time to the more symmetrical cri- 


Me 


3 


su 
PAURT 
Z ig 
kbs cosas 


nn 
» 





sae 


b 





s K 

Fig. 342. ORDOVICIAN ECHINODERMS: 4a, Comarocystis punctatus Billings; }, 
Lepidodiscus cincinnatiensis (Roemer); c, Plewrocystis filitextus Billings; d, Ecteno- 
crinus grandis (Hall); e, Dendrocrinus polydactylus (Shumard) ; f, Hybocrinus tumidus 
Billings; g, Lepadocystis mooret (Meek); h, Carabocrinus vancortlandti Billings; 7, 
Archeocrinus desideratus Billings; 7, Glyptocrinus decadactylus Hall; k, Anomalo- 
crinus incurvus M. and W.; 1, Paleaster simplex Miller. a, b and ¢ are cystoids, d-k 
are crinoids, / is a star-fish. 
face uppermost and fixed to the sea-bottom by a calcareous stem 
attached to the center of the back. Crinoids so closely re- 
sembled a flower in form, that the familiar name ‘‘sea-lily”’ is not 
inappropriate. 

Corals are few in the lower part of the system, and though more 
abundant in higher beds, are nowhere a leading part of the fauna. 


LIFE 385 





Fig. 343. ORDOVICIAN GRAPTOLITES: a, Dichograptus octobrachiatus (Hall); 
b, Reteograptus eucharis Hall; c, Phyllograptus ilicifolius Hall; d, Diplograptus pristis 
(Hall) (restored by Ruedemann); e, Tetragraptus fruticosus (Hall); f, Climacograptus 
bicornis (Hall); g, Didymograptus nitidus Hall; h, Tetragraptus bigsbyi (Hall); 
1, Phyllograptus typus Hall; 7, Holograptus richardsoni (Hall). 


Most of them belonged to the simpler horn-shaped type (Fig. 344, 
_a), but compound and colonial 
corals were present. The most 
important development of the 
ceelenterates was the rise of the 
graptolites (Fig. 343), whose im- 
portant function in correlation 
has been referred to (p. 377). 
Some sponges (Fig. 345) at- 
tained notable size. The record 
of annelids (worms) is oni Fig. 344. ORDOVICIAN CoRALS: 
meager than in the Cambrian, a, Sireptelasma corniculum Hall; 6, 
perhaps because the calcareous Columnaria alveolata Goldf. Both 


+s simple and compound corals lived 
sea-bottom of the Ordovician was}; thew idid Hot form ereaereets ay 


less congenial to them than the _ in later periods. 





386 ORDOVICIAN PERIOD 





a 


is 
i 





- 
fe) 






7 


Lt 








e 


Fig. 345. ORDOVICIAN SPONGES: a, Receptaculites occidentalis Salter; 6, 
Brachiospongia digitata Beecher; c, Archecyathus minganensis Billings; d, Proto- 
spongia maculosa U. and E.; e, Ischadites, species undetermined. Receptaculites 
and Jschadites were formerly regarded as giant foraminifers. 


Cambrian sands. They are represented by burrows and by 
teeth (Fig. 346). 

Fragmentary fossils of fishes constitute the most striking innova- 
tion in the record of the marine life of the period. These have 





d e f 


Fig. 346. JAws oF OrpovIcIAN ANNELIDS, highly magnified: a, Arabellites 
cornutus Hinde; b, Glycerites sulcatus Hinde; c, Eunicites gracilis Hinde; d, Arabellites 
ovali, Hinde; e, Eunicites varians (Grinnell); f, Oenonites rostratus Hinde. 


been found in a few localities only, notably near Canyon City, Colo., 
and in the Bighorn Mountains of Wyoming. As in the Cambrian, a 
vast supply of unrecorded vegetation must be postulated to have sup- 
plied food for the animals. To provide for organisms that preyed 
upon one another in succession, from plants up to the master forms of 
the predaceous animals, there were doubtless many species not now 
known. ‘The fact that vegetal tissues are not found among fossils, 
save in rare cases, probably signifies that the bacteria concerned in 
the decomposition of organic matter were abundant. 

General advancement. It seems clear that the adaptation of the 
various forms of life to one another and to their physical environ- 
ment had reached a higher stage of adjustment than in the Cam- 


brian, an adjustment not greatly inferior to that which now prevails 
among the corresponding orders. Higher types within the same 
orders have been developed since in many cases, but some of the 
Ordovician forms have since suffered degeneration. The Ordovician 
ancestors of the barnacle, for example, free-moving, active forms, 
were doubtless superior to their sessile descendants of ill-repute. 

Land life. But few relics of land plants, and these somewhat 
doubtful, have been found in the system, and they reveal but little. 

The oldest relic of insect life now known is a rather obscure wing 
found in the shales of the Upper Ordovician of Sweden. It is re- 
ferred to the order of Hemiptera (bugs). The existence of flying 
insects implies the presence of vegetation, and of atmospheric 
conditions suited to active, air-breathing organisms. 


Succession of Faunas 


There was a succession of Ordovician faunas, somewhat unlike 
one another, just as there was a succession of Cambrian faunas. 
These may be distinguished roughly as the Lower, Middle, and 
Upper Ordovician faunas. In some places, the late Cambrian 
and early Ordovician faunas merge into one another without 
sharp definition. In general, the Mid-Ordovician fauna was 
more prolific than that which preceded, if we may judge from 
the fossils. The Mid-Ordovician fauna, too, was distinctly cos- 
mopolitan. The Upper Ordovician fauna was similar to its prede- 
cessor, from which it descended, but clear-water forms were less 
dominant. 

Map work. See note at end of last chapter. The folios serviceable for the 


Cambrian system are serviceable also for the Ordovician. See also Exercise [X 
in Laboratory Exercises in Structural and Historical Geology. 


CHAPTER XVII 
THE SILURIAN (UPPER SILURIAN) PERIOD 


FORMATIONS AND PHYSICAL HISTORY 


The changes which brought the Ordovician period to a close 
marked also the inauguration of the Silurian. These changes 
included movements which affected (1) small areas intensely, and 
(2) broad areas slightly. From the standpoint of continental his- 
tory, the latter were the more important. These changes were 
accomplished slowly, and after they had taken place, the area of 
land in North America was greater than at any time since the early 
Cambrian. The increase in land meant lengthened streams, and 
presumably increased erosion. 

It is safe to assume that at the opening of the period clastic 
sediments were accumulating about the immediate borders of the 
lands, and as far out as waves and currents were able to transport 
detritus, and that elsewhere sediments of organic origin were rela- 
tively more important. Though sedimentation was interrupted in 
regions which emerged from the sea at the close of the Ordovician 
period, the interruption was not universal, and Silurian strata are 
locally conformable on the Ordovician in the continents, and gen- 
erally, it is presumed, in the ocean basins. 

In New York, the Silurian is subdivided as follows!: 


Manlius limestone 


Cayugan Rondout waterlime 
(Upper Silurian) Cobleskill limestone 
Salina beds 
Guelph dolomite 
Silurian Niagaran Lockport limestone 
(Middle Silurian) | Rochester shale 
Clinton beds 
Oswegan Medina sandstone 
(Lower Silurian) 1 Oneida conglomerate (and perhaps the 
Richmond beds) 


1 There is infelicity in the use of the terms Lower, Middle, and Upper Silurian 
for the subdivisions of the system, since Lower Silurian was long used as a synonym 
for Ordovician, and Upper Silurian for Silurian, as that term is here employed. 


388 


FORMATIONS AND PHYSICAL HISTORY 389 


Fach of the three series is made up of several formations, but the 
subdivisions of one place do not fit another. A brief sketch of the 
nature and distribution of the principal subdivisions of the system 
affords an outline of the history of the continent during the period. 


Silurian of the East 


Oswegan series. This series is known chiefly in New York,! 
both the Oneida and the Medina formations appearing at the surface 
south of Lake Ontario. Their equivalents may recur in the western 
part of the Appalachians farther south. The Oneida consists of 
conglomerate and sandstone, and the Medina of sandstone and shale. 
The sediments of these formations appear to have been deposited in 
a shallow interior sea, as shown by fossils, and by the cross-bedding, 
ripple-marks, etc., which affect them. Both formations are prob- 
ably continuous beneath younger strata over considerable areas 
south of Lake Ontario, and west of the Appalachians,? and the’ 
Medina is more wide-spread than the Oneida. 

Niagaran series. The Clinton formation overlies the Medina 
conformably, but has a wider distribution, extending westward to 
Lake Huron and Indiana, and perhaps to the Mississippi, and south- 
ward, in the Appalachians, to Alabama and Georgia. Beds of 
Clinton age have been recognized in Nova Scotia and at a few other 
places northeast of the United States, where marine sedimentation 
was probably continuous through the Ordovician and Silurian 
periods. In the Appalachian Mountains, the formation is largely 
of sandstone and shale. In western New York and farther west, 
much of it is limestone. 

One of the features of the formation is its zron ore, generally ia 
the form of hematite (Fe.O;). The ore is known at many points 
between New York and Alabama, as far west as Wisconsin, and in 
Nova Scotia. It is interstratified with other beds of the formation, 
and is usually believed to have been accumulated by chemical 
precipitation in lagoons or marshy flats. 

The Niagara formation (subdivided in New York, p. 388), 
extends farther west than any of the preceding Silurian formations, 
showing that the submergence begun earlier still continued in the 
upper Mississippi basin. ‘The falls of Niagara River are over the 

1The formations of eastern New York and New Jersey formerly classed as 


Oneida, are of Salina age. Hartnagle, Bull. 107, N. Y. State Mus. 
2 Perhaps the Richmond beds, the Maquoketa shales, etc. 


39° 


SILURIAN PERIOD 


“<*Teseseg 





Map showing the outcrops of the Niagara formation, and at the same 


Fig. 347. 
time the general relations of land and water during the Niagara epoch. The vari- 
ous sorts of shading on the map correspond with those on earlier maps (see p. 345). 


FORMATIONS AND PHYSICAL HISTORY 301 


limestone of this series (Fig. 76). North of Missouri, the formation 
is not known far west of the Mississippi, but it extends into Mis- 
souri, Arkansas, and perhaps even to the Arbuckle Mountains of 
Oklahoma. Itis found alsoin western Texas. The southern border 
of the interior sea in which this formation was deposited is not 
known, but it may have been separated from the Gulf of Mexico, 
by a land barrier (Fig. 347). 

A significant feature of the distribution of the Niagara forma- 
tion is its great development in northerly latitudes (p. 390). The 
known patches in the north appear to be remnants of a once continu- 
ous formation, and since the fossils are much like those of northern 
Europe, it is inferred that there was shallow-water connection be- 
tween the Mississippi basin and northern Europe by way of the 
Arctic islands, which permitted the intermigration of the shallow- 
water sea-life of the two regions. 

East of the Appalachians and west of the Mississippi the distri- 
bution of Niagaran strata is not known in detail. Their exact 
equivalents have not been identified in the West. West of New 
York the formation is mainly limestone. It is the oldest formation 
in which well-developed coral reefs have been identified, though 
coral-secreting polyps had lived 
before (p. 385). Reefs are 
known in eastern Wisconsin, Z 
Indiana, and elsewhere. ESA NTR 

In the east where the Ni- eh 
agara is known, it has a thick- RRR Sewara 
ness of but 100 to 300 feet, 
while in Wisconsin it attains a 
maximum of 800 feet (perhaps 
including some Clinton), all of which is limestone. While the 
Niagaran beds of the interior are in general nearly horizontal, they 
are domed in many places, giving the beds a high dip (Fig. 348), 
as at various points about the south end of Lake Michigan. 

Cayugan series. The Salina formation, which overlies the Ni- 
agaran series in parts of New York, Pennsylvania, Ohio, Michigan, 
and Ontario, is much less widespread, indicating the emergence of a 
considerable area in the Mississippi basin at the close of the Niagaran 
epoch. The formation embraces all common sorts of sedimentary 
rock, and in addition rock salt, and gypsum. Shale is the most 
abundant sort of rock, and seems to have originated after the fashion 




















SATIS 
PSST RNS 
7 ASU ys 
Fig. 348. The Wabash dome in the 
Niagara limestone. (Kindle.) 









302 SILURIAN PERIOD 


of shales in general, but the fewness of its fossils points to deposition 
under conditions unfavorable for life. The salt is widely distrib- 
uted. In New York alone it occurs at many points within an area 
of 9,000 to 10,000 square miles. Single beds of it are locally 40 to 
80 feet thick. In places, several beds occur one above another, 
interstratified with other kinds of rock, and their aggregate thickness 
reaches as much as too feet in places. Near Cleveland, four salt 
beds, 50 feet and less in thickness, are interstratified with 500 feet 
of shales. 

The salt beds seem to imply the existence of great lagoons or 
inclosed seas. Deposits of gypsum are made under about the same 
conditions as salt beds. Had the climate of this region been as moist 
as now, lagoons could not have been abnormally saline. Occasional 
incursions of the sea, bringing in new supplies of salt water, followed 
by periods when the lagoons were cut off from the sea, and when they 
suffered rapid evaporation, would seem to meet the conditions de- 
manded for the salt. So also would a slight continuous connection 
with the sea, such that the inflow of sea-water into the basin was less 
than the excess of evaporation over precipitation. A bed of salt 40 
feet thick implies the evaporation of some 3,000 feet of normal sea- 
water. Much of the commercial salt which comes from New York 
is derived from the waters of salt wells. 

The limestone of the Salina contains few fossils, and has been 
thought to be a chemical precipitate. The relations of limestones, 
shales, and salt beds are such as to indicate that the sites where these 
several sorts of rock were formed shifted from time to time, as if by 
gentle crustal warping. 

Above the Salina proper of New York, there is a thin (150 feet 
maximum) series of limestones (p. 388), more widespread than the 
Salina. Its equivalent extends westward through Ohio to Indiana 
and Wisconsin. Both its distribution and its character show that 
the eastern interior was more generally submerged than during the 
deposition of the salt-bearing series which preceded. 

The Helderberg formation, formerly regarded as a part of the 
Silurian system, is here classed with the Devonian. 

Summary. As in the preceding systems of the Paleozoic, the 
greatest thicknesses of Silurian strata (estimated at about 5,000 feet, 
maximum) are in the Appalachian region. Over the interior, the 
system is measured by hundreds of feet rather than by thousands. 
In keeping with these variations of thickness, the system is largely 


FORMATIONS AND PHYSICAL HISTORY 393 


of clastic sediments in the Appalachian belt, while in the interior it 
is largely of limestone. The site of sedimentation in the east was a 
sort of trovzh (the Appalachian trough) shut off from free communi- 
cation with the interior sea, but connected narrowly with the 
Atlantic, perhaps by way of the present Chesapeake region. Since 
most of the sediments of this trough were deposited in shallow-water, 
they are thought to indicate that the trough was sinking at a rate 
comparable to that at which the sediments accumulated. With the 
down-warping of the trough, there may have been up-warping of 
the adjacent lands to the east, which supplied the sediments. 

The history of the Silurian period, as now understood, involves, 
(1) a general submergence of the eastern part of the United States 
west of Appalachia, by which the sea became more and more wide- 
spread until the close of the Niagaran epoch; (2) a partial with- 
drawal of the sea from the same area in the Salina epoch; and 
(3) an extension of the sea at the close of that epoch. 


Silurian of the West 


At various points in the West, there are sedimentary beds, poor 
in fossils, between the known Ordovician below and the Devonian 
above. The character of the fossils being indecisive in many places, 
the age of the beds is open to question. Some of them may be 
Silurian. If the Silurian is really absent from all the areas where 
its presence is not now known, it would appear that a large part of 
western North America was land during the Silurian period. Silu- 
rian beds are however known in Southern California, Nevada, Utah, 
and Alaska, and perhaps in the Canadian Rockies, and their dis- 
tribution may be more widespread than has been supposed. 


General Considerations 


Igneous rocks. At few points in North America have igneous 
rocks of Silurian age been identified. Silurian formations are locally 
affected by intrusions, but their dates are generally uncertain. 
Some of the igneous rocks of New Brunswick are thought to be of 
Silurian age, and perhaps some of those of Nova Scotia and Maine. 

Close of the period. The geographic changes at the close of the 
Silurian were less than those at the close of the Ordovician, and 
the system is less distinctly separated from the Devonian above than 
from the Ordovician below. 

Climate and duration. There is nothing to indicate great 


394 SILURIAN PERIOD 


diversity of temperature in the Silurian period, and much to suggest 
that uniformity extended through great ranges of latitude, for the 
fossils of warm-temperate regions are in part the same as those in 
Arctic regions. Some regions appear to have been temporarily very 
arid. This probably was one of the shorter Paleozoic periods. 


Foreign 


In Europe the Silurian strata have a distribution similar to that 
of the Ordovician, though they are wanting in some regions where 
the latter are present. The fact that the Silurian strata do not 
appear at the surface over wide areas does not indicate their general 
absence, so much as their widespread concealment. In most of 
the northern part of Europe, outside of Britain, the system has been 
little deformed. In contrast with the Silurian rocks of the northern 
province, those of the southern are much deformed. 

There were important geographic changes in some parts of 
Europe at the close of the period, as shown by the unconformity 
between the Silurian and Devonian systems in some places (Great 
Britain and Ireland). In some parts of western Europe there were 
overthrust faults of great extent. Locally (Scotland) the thrust was 
as much as ten miles,! and had for a result, the thrusting of Cam- 
brian and even Archean formations, over the Silurian. 

The Ordovician and Silurian of other continents have not been 
generally distinguished. Equivalents of the two systems probably 
occur in all the less well-known continents. 


LIFE 

The extensive withdrawal of the sea from North America at 
the close of the Ordovician period reduced the area of shallow water. 
available for the life which needed it. The severe repressive evolu- 
tion which followed was the great biological feature of the transition 
from the Ordovician to the Silurian. With the re-invasion of the 
interior by the mid-Silurian sea, there followed an expansional 
evolution of the shallow-water fauna which constitutes the great 
biological feature of the middle of the period. ‘Toward its close, 
there was restriction of the epicontinental sea, complicated with 
intense salinity in the eastern interior, and there followed a second 
repressive evolution through which the Silurian fauna passed into 
the Devonian. 

Theoretically, the history of the land life should have been the 

‘Quart. Jour. Geol. Soc., 1884 and 1888. 


LIFE 3905 


reciprocal of that of the sea; for as the sea contracted, the land 
expanded, and an expansion of land life should have run hand in 
hand with the restriction of sea life. The record of land life is 
too meager to demonstrate that this was the fact. In so far as the 
climate was arid, it was unfavorable for abundant land life. 
Transition from the Ordovician. Of the shallow-water life of the 


early Silurian there is but 
meager record. The eastern 
shore of the continent was 
then farther east than now, 
and the deposits there are 
buried and __ inaccessible. 
The western border may 
have been submerged, but 
the fauna there is little 
known. 

In addition to the lessened 
area available for shallow- 
water life, the conditions 
probably were less favorable 
than before. The increased 
detritus brought to the sea 
probably inhibited some 
forms of life, injured others, 
and helped but few. Some 
of the basins and bays were 
doubtless too fresh and 
some too salt. These gen- 
eral considerations may ex- 
plain the meagerness of the 
faunas of the early Silurian 
strata. But conditions were 
not adverse everywhere. In 
the Gulf of St. Lawrence, 
Ordovician species lived on 
for varying lengths of time, 
and mingled with Silurian 
species as they developed, 
and so recorded the transi- 
tion. 





Fig. 349. SILURIAN ECHINODERMS: 4, 
Eucalyptocrinus crassus Hall, a complete 
crinoid, showing roots, stem, and body; 3), 
Holocystites adiapatus Miller, a cystoid with 
irregularly arranged plates and scattered 
pores; c, Lecanocrinus macropetalus Hall, an 
articulate crinoid; d, Troostocrinus reinwardti 
(Troost), showing the typical bud-like form 
of a blastoid; e, Caryocrinus ornatus Say, a 
cystoid with regularly arranged body plates. 
Pores in radiating lines from centers of plates. 


306 SILURIAN PERIOD 


Mid-Silurian fauna. As the sea slowly overspread the continent 
toward the middle of the period, the increasing room and more con- 
genial conditions for most forms of shallow-water life resulted in an 
expansional evolution which produced the Niagara fauna. The 
families and classes were much the same as in the Ordovician period, 
but most of the genera were new, and nearly all the species. In 
general there was a biological advance, though this was not true of 
all classes. Only the more conspicuous features of the changes will 
be noted. 

A distinguishing feature of the Silurian fauna was the rich and 
varied development of the echinoderms, especially the crinoids (Fig. 
349). They attained such abundance in certain localities that their 
fragments form the larger part of the limestone. ‘These spots were 
veritable ‘‘flower-beds”’ of ‘‘stone lilies,” where beautiful and varied 
forms grew in groves, as it were. Cystoids were still abundant, and 
blastoids appear for the first time. Starfishes, serpent-stars and 
echinoids were unimportant elements in the fauna. 





a Fe 

Fig. 350. SILURIAN BrRaAcuHtopops: a, Pentamerus oblongus Murch., lateral 
view of an interior cast; b, Trimerella acuminata Bill., the interior view of a pedicle 
or ventral valve, showing the elevated platform for muscular attachment excavated 
beneath; c, Spirifer niagarensis (Con.), exterior view of the brachial or dorsal valve, 
with the cardinal area and beak of the pedicle valve showing above; d, Chonetes 
cornutus (Hall), exterior view of ventral valve, showing the cardinal spines; e, 
Trimerella ohioensis Meek, the internal cast of a highly differentiated inarticulate 
brachiopod, showing the fingerlike casts of the excavations beneath the elevated 
muscular platforms; f, Stropheodonta profunda Hall, interior of the ventral valve, 
showing muscular impressions; g, Spirifer radiatus Sow.; h, Streptis grayi (Dav.), 
exterior view of the brachial valve, showing the cardinal area and beak of the 
opposite valve, and the peculiar twisted form of the shell, 


BEE 397 


Brachiopods (Fig. 350) lived on from the Ordovician with no loss 
of prestige, though most species and many genera were new. The 
Silurian brachiopods showed some notable advances in structure. 
On the whole they were more robust and gave more obvious signs of 
abounding vitality than before; but along with the progressive 
developments there were some retrograde modifications. 

Among mollusks, cephalopods appear to have remained the most 
powerful inhabitants of the seas. Straight forms were still com- 
mon, but curved and coiled ones were more numerous. Their 
shells were more highly ornamented than before, though still plain 
in comparison with some of their successors. The apertures of the 
shells of most Ordovician species were circular or oval, but in the 
Silurian species many of them were curiously constricted (0d, Fig. 
351), especially among the small curved and coiled species. The 





Fig. 351. SrLurtrAN Mottusks: a and b are cephalopods. a, Orthoceras 
annulatum Sow., a straight chambered shell with annulations; b, Phragmoceras 
nestor Hall, lateral view of a curved chambered shell with peculiar constricted aper- 
ture. c,d, and ¢ are gastropods; c, Loxonema leda Hall; d, Platyostoma niagarensis 
Hall; e, Subulites ventricocus Hall; f, Pterinea emacerata (Con.), exterior view of 
left valve of a pelecypod. 
constriction appears to have been a protective device. Gastropods, 
fairly well represented in the Cambrian period and amply in the 
Ordovician, did not increase greatly in the Silurian. They show 
advance in the preponderance of elevated spires, in increased variety 
of form, and some of them in greater size; but the older types were 
still plentiful. Pelecypods (f, Fig. 351) were not more plentiful than 


in the Ordovician, 


398 SILURIAN PERIOD 


The prominence gained by corals (polyps) in suitable situations 
is one of the notable features of the Silurian fauna. In the Ordovi- 
cian period, simple forms predominated over compound, but the 
ratio was now reversed. Among the notable types was the chain 





Fig. 352. SrLuRIAN CoRALS AND BRYOZOANS: a-e are corals. a, Favosites 
occidens Whit.; b, Syringopora verticillata Goldf.; c, Halysites catenulatus Linn.; d, 
Goniophyllum pyramidale (His.); e, Zaphrentis wmbonata Roming. Bryozoans, f 
and g, Fenestella parvulipora Hall. 


coral (c, Fig. 352), which had appeared in the Ordovician; the honey- 
comb coral (a); the organ-pipe coral (6); and the cup coral (e). A 
most peculiar simple coral was quadrangular, and its top provided 
with a cover (operculum) of four triangular plates hinged to the four 
sides of the cup’s margin. When closed they formed a pyramid over 
the cup (d, Fig. 352, only two opercular plates shown). This was a 
protective device unknown among modern corals. With their 


LIFE 399 


increase in abundance, corals acquired the habit of associating them- 
selves together. This resulted in the formation of reefs. The 
known reefs appear to have been of the barrier type formed some 
distance from shore. The reef-forming habit appears to have been 
local rather than general. 

Among ?rilobites, no new families appeared, though there were 
some new genera and many new species; but the new forms did not 





G 


Fig. 353. SILURIAN,TRILOBITES: a, Spherexochus mirus Bey., dorsal view; 8, 
Staurocephalus murchisoni Barr, dorsal view, showing the peculiar globular anterior 
prolongation of the head; c, Deiphon forbesi Barr, dorsal view of a peculiar trilobite 
having the pleural lobesimuch reduced; d, Calymene niagarensis Hall, dorsal view 
of one of the commonest Silurian trilobites; e, Cyphaspis christyi Hall, dorsal view. 


offset the disappearance of old ones, and the class, though still 
important, had already begun its decline. The highest forms were, 
however, equal, if not superior, to any that preceded. . 

Sponges flourished. ‘There was a prolific field of them in western 
Tennessee, where the conditions were not only congenial to their 
growth, but favorable for their preservation. Graptolites had lost 
the importance they had in Ordovician times, and by the end of the 
period neared extinction. Sea-worms are recorded through their 
jaws, tracks, and burrows, and by the calcareous tubes which some 
of them secreted. 

In the earliersand mid-Silurian deposits few relics of fishes have 
been found, and these few are very imperfect; but in the upper part 
of the system their remains are not rare. 


400 SILURIAN PERIOD 


Knowledge of Silurian marine plant life is meager. While it 
must have been abundant theoretically, only obscure markings have 
been found, and their interpretation is more or less doubtful. 

Late Silurian fauna. Following the luxuriant life of the mid- 
Silurian epoch, there came, in North America at least, a notable 
decline, due to the withdrawal of the epicontinental waters from the 





Fig. 354. Receptaculites owent Hall; ‘lead coral” or ‘‘sunflower’’; original 9l4 
inches in diameter. 
- larger part of the interior, and to the conversion of the remainder into 
an excessively salt sea, in the deposits of which few fossils are found. 
At the very close of the period there was in the Salina basin a gradual 
return of conditions hospitable to life. The fauna of these late 
Silurian beds is limited, and radically unlike that which preceded. 
Most of the familiar marine types are absent from the later fauna, 
and its signal feature is an abundance of arthropods (p. 686) of types 
barely represented before. The most characteristic of these were 
the great Eurypterus and the still more gigantic, Pterygotus (Fig. 
355,a@ and 0). The former reached a length of a foot and a half or 
more, and in the next period the latter attained a length of over six 


LIFE 401 


feet. These giants among their kind were aquatic, but whether 
inhabitants of salt or fresh water is not certain. 

Mollusks, crinoids, corals, and similar marine forms are almost 
entirely absent from the fauna of the Waterlime. The few brachio- 
pods found are usually pauperitic, as though they lived in uncon- 
genial conditions. 

It was at this time that the earliest known scorpions, kin of the 
eurypterids, appeared both in America and Europe. The European 


Pes 
y 
uN 


aN 


SE 


SEY a Vas 
— | iH) 






























































































































Fig. 355. Arthropods, other than Trilobites: a, Eurypterus fischeri Eich.; 
L, Plerygotus anglicus Agass.; c, Paleophonus caledonicus Hunter. 


forms have been thought to be land species, though this has been 
questioned. The stings and poison glands have been identified, 
and the significant name, Palewophonus, ‘‘ancient murderer,”’ 
applied in consequence (Fig. 355, c). The American species have 
been thought to be aquatic. 

The presence of fishes emphasizes the peculiarities of this fauna. 
Except for their occurrence at a few points in the Rocky Mountains 
in the Ordovician, fish remains have not been found in America 
until this stage. In Europe a few fishes appear somewhat earlier, 
but nearly all fish remains of the period yet found are in the upper- 
most horizons of the Silurian, or in deposits that form the transition 
to the Devonian, where they are associated with eurypterids, land 
plants, and marine invertebrates. 


CHAPTER XVIII 
THE DEVONIAN PERIOD 


FORMATIONS AND PHYSICAL HISTORY 

The most important physical events of the Devonian period in 
North America were the invasions of large areas of the continent by 
the sea, which came in from different directions in successive epochs. 
The invasions appear to have been from the east, Pe the north, 
the south, and the northwest, in turn. 

Early in the Devonian petiods the sea covered the present area 
of land to some such extent as shown in Fig. 356. During the 
period, there were notable changes in the relations of land and 
water, with corresponding changes in the life of the period. 

The subdivisions of the Devonian system now recognized in 
New York (where the Devonian is best known) are as follows: 


{ Chautauquan Chemung and Catskill 
Portage beds 
| Senecan Genesee shale 
. Tully limestone 
Hamilton shale 
Marcellus shale 


{ Onondaga limestone 


Upper Devonian 


Erian 


Middle Devonian 


Devonian | Ulsterian Schoharie grit 


Esopus grit 


Oriskanian Oriskany beds 
{ Kingston beds 
Lower Devonian Felderhersinn Becraft limestone 
New Scotland beds 
L Coeymans limestone 
The subdivisions of the last two columns do not fit regions 
remote from New York, though the names, Helderberg (or Helder- 
bergian), Oriskany, Onondaga (Corniferous), Hamilton, Portage, 
and Chemung, have rather wide application. 


Devonian of the East 


The Lower Devonian. The known Helderbergian series, Thaly | 
limestone, is known in (1) the northeast, (2) the Appalachian belt, 
402 


FORMATIONS AND PHYSICAL HISTORY 403 





Fig. 356. Map of North America, showing the outcrops of the Helderberg 
formation and the general relations of land and water during the Helderberg epoch. 
The conventions are the same as in the earlier maps of the series. 


404 DEVONIAN PERIOD 


and (3) the lower Mississippi basin (Fig. 356). The Oriskany 
formation, chiefly sandstone in the east, has a similar, but some- 
what wider, distribution. It is best known in the northern Appa- 
lachian region. From the vicinity of Cumberland, Md., where it has 
a thickness of a few hundred feet, it thins to the northeast and south- 
west, and loses its most distinctive faunal characteristics as it thins. 

The Middle Devonian. The Middle Devonian is more wide- 
spread than the Lower and its most important formations are the 
Onondaga and Hamilton. The Onondaga limestone is found from 
New York to the Mississippi (Fig. 357), resting on Silurian beds 
with little evidence of unconformity. The epicontinental sea in 
which it was formed was relatively clear and shallow, as shown by 
the composition of the rock and its fossils. In many places the 
limestone is rich in coral, and locally the coral-reef structure is 
shown perfectly. This is true, for example, at the rapids of the 
Ohio near Louisville. The formation is rarely more than 100 to 200 
feet. In northern New England and Canada, the equivalent of 
this formation has a distribution similar to that of the Lower Devo- 
nian. It occurs also on the west side of the south end of Hudson 
Bay, and the beds here may have been connected formerly with 
equivalent formations in the interior of the United States. 

Following the Onondagan epoch of clear seas, conditions changed 
so as to give origin to deposits of mud in many places where lime- 
stone had been forming. These mud beds, now consolidated, con- 
stitute the Marcellus and Hamilton formations of New York (p. 402). 
In the interior, where there is more limestone, the equivalents of 
the two formations are commonly grouped together under the name 
Hamilton, or given local names. 

Considerable areas in the southern and in the northwestern parts 
of the Mississippi basin which had been land earlier, appear to have 
been submerged at this time (Fig. 358), for the Hamilton formation 
appears to overlap its predecessor in these directions, resting on the 
Silurian. The spread of the sea at this time seems also to have 
submerged areas in the southern Appalachians which had been land 
since the close of the Ordovician. Connection may have been made 
at this time between the interior sea and the Gulf of Mexico, allow- 
ing shallow-water species of animals to migrate from the south into 
the Mississippi basin. 

The conditions for the origin of the Hamilton shales would seem 
to be met if the surrounding lands (Appalachia and lands north of 


FORMATIONS AND PHYSICAL HISTORY 405 


im eenee ed. 





Fig. 357. Map of North America, showing the outcrops of the Onondaga forma- 
tion, and the general relations of land and water during the Onondaga epoch. The 
Devonian at the northwest is not all Onondagan. Note the interrogation marks in 
the lower Mississippi valley. 


406 DEVONIAN PERIOD 


the interior sea), after standing low while the Onondaga limestone 
was making, were elevated, or were less protected by vegetation, 
or subjected to more concentrated or spasmodic precipitation 
during the Hamilton epoch. The land formations might then have 
been undergoing decay during the Onondaga epoch, though the 
products of decay were not removed. Under the changed condi- 
tions postulated, there would have been opportunity for their 
transportation and deposition. 

In the east, where the series is mainly clastic, it reaches a thick- 
ness of 1,500 to 5,000 feet (Pennsylvania); but in the interior, where 
it contains more limestone, it is much thinner. 

The Upper Devonian. The Upper Devonian has a distribution 
(Fig. 359) similar to that of the Middle, though somewhat more 
widespread.. The Upper Devonian is more distinct from the 
Middle than the Middle is from the Lower, and is somewhat closely 
connected with the lower part of the succeeding system.! An un- 
comformity appears at the base of the Upper Devonian in some 
places, and in others the series overlaps other Devonian formations, 
resting on the Ordovician. 

The Senecan series of New York consists of various thin forma- 
tions (p. 402), chiefly clastic, of shallow-water and terrestrial origin. 
The Chemung formation of western New York is similar to the Sene- 
can formation, though more sandy, or even conglomeratic. The 
Catskill formation of the Catskill region consists of red shales and 
sandstones, which appear to be, in a general way, the time-equiva- 
lents of the Chemung. In some places the Catskill beds may 
represent more than the Upper Devonian, and in others less. They 
are poor in fossils, and those known are partly, if not wholly, fresh- 
and brackish-water forms. Hence it is inferred that the Catskill 
region was so far shut off from the ocean as not to afford the condi- 
‘ tions necessary for marine life. Redness characterizes many forma- 
tions made in inclosed or partially inclosed basins. Outside the 
Catskill region local beds of red sandstone suggest that similar 
conditions of deposition existed now and then farther west. ! 

The thickness of the Upper Devonian in central and western 
New York approaches 4,000 feet, and is even more in Pennsylvania 
and Maryland. In Ohio the equivalent series (Black, or Ohio 


1 Ulrich has recently proposed grouping the Upper Devonian with the lower 
part of the Mississippian, as a new system, under the name of Waverlyan. Bull. 
Geol. Soc. Am., Vol. XXII. 


etal ta 





Fig. 358. Map of North America showing the outcrops of the Hamilton forma- 
tion and the general relations of land and water during the Hamilton epoch. The 
black area at the north probably includes Lower and Upper Devonian as well as 
Middle. Compare Figs. 356 and 357. The conventions are the same as in earlier 
maps. 


408 DEVONIAN PERIOD 


shale) has a maximum thickness of 2,600 feet, and thins notably to 
the north and west, to a few hundred, and in places even to a few 
score feet. Different names are applied to equivalent formations 
in various localities. 


Devonian of the West 


Most of the Great Plains region is without Devonian formations, 
so far as known, and so is inferred to have been land; but the Helder- 
berg formation is present in the Arbuckle Mountains of Oklahoma, 
and probably in southwestern Texas. The system has little devel- 
opment in the Rocky Mountains, but is widespread between the 
Rockies and the Sierras, though its outcrops are not extensive. In 
some places, as about Globe, Arizona, the system is much faulted 
and affected by igneous rock;! in others it is bounded by unconformi- 
ties, both below and above, while in still others its limits are not 
defined sharply. Where subdivisions of the system have been made, 
they are not correlated with those of the east. In the Great Basin 
region, both Onondagan and Hamilton types of fossils are found. 
Their testimony is to the effect that the basin region was not con- 
nected with the eastern interior sea in such a way as to allow the free 
intermigration of marine life. The system is said to be 8,000 feet 
thick in parts of Nevada, and 2,400 feet in the Wasatch Mountains; 
but in the Yellowstone Park, it is only 160 feet thick, and not divisi- 
ble into distinct formations. In the western interior generally, 
limestone is the dominant formation. 

Devonian formations are known in both northern and southern 
California, and may be present in many places where the rocks are 
metamorphosed past identification. The system also is represented 
in widely separated parts of Alaska. The Devonian faunas of the 
coastal region, like those of the Great Basin, are Eurasian in their 
affinities. 

Middle Devonian in the northwest. A considerable area of De- 
vonian which has sometimes been called Hamilton is found in the 
basin of the Mackenzie River and south to Manitoba. The arm of 
the sea in which these Devonian beds accumulated appears to have 
extended as far south as northern Missouri (Fig. 358). The fossils 
of this northwestern Devonian are different from those of the 
Hamilton fauna east of the Mississippi, and if the beds of the two 


1 Ransome, Professional Paper No. 12, and Bisbee folio, U. S. Geol. Surv., 
pp. 39-46. 


FORMATIONS AND PHYSICAL HISTORY 


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issippi basin.) 


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the lower M 


4xO DEVONIAN PERIOD 


regions were contemporaneous, they were probably deposited in 
waters which were not connected (Fig. 358). Toward the end of 
the Hamilton epoch, the barrier which separated their waters seems 
tc have been removed sufficiently to allow the waters and the life 
on opposite sides to mingle freely (Fig. 359). 


General Considerations 

Outcrops. While the Devonian system is widely distributed in 

North America, it does not appear at the surface in large areas. 
_ The reasons are substantially the same as those for the limited 
exposures of earlier systems. The removal of Devonian from areas 
it once covered is oddly shown near Chi- 
cago, where a small remnant of Devonian 
TTI sediment has been found in a fissure in the 
Fig. 360. Figure illus Niagara limestone, as shown in Fig. 360. 
trating the occurrence of The limestone was apparently fissured 
remnants of Devonian ma- , ‘ 
terial in fissures in Niagara before the Devonian sediments were de- 
limestone, near Eliahurst posited upon it. Portions of the sedi- 
(Cook Co.), Illinois. ments fell into an open fissure, carrying 
with them distinctive fossils (fish teeth). In this protected position, 
the fossils escaped removal. 

Igneous rocks. Igneous rocks have little representation in most 
parts of the system in North America, but in Nova Scotia, New 
Brunswick, and Maine, and at some points in the west, there are 
igneous rocks which appear to be of this age. In many places in 
the west, Devonian strata have been affected by dikes and intru- 
sions of later times. 

Close. The general quiet which had prevailed during the period 
seems not to have ended at its close. Only in the eastern part of the 
continent, so far as now known, in Maine, Nova Scotia, New Brun- 
swick, and the adjacent region to the north were Devonian strata , 
notably disturbed at the close of the period. Elsewhere the for- 
mations of the younger system rest on those of the older without 
stratigraphic break. 





Economic Products 


The Upper Devonian is the chief source of oz! and gas in western 
Pennsylvania and southwestern New York, and is one of the sources 
in West Virginia. ‘he Middle Devonian is oil-producing in On- 
tario. Within the regions of their occurrence, oil and gas are more 
likely to be founda under low anticlines than in other positions, 


FORMATIONS AND PHYSICAL HISTORY Alt 


apparently for the reason that anticlines furnish an inverted basin 
capable of holding these substances against the pressure of the 







(e] 
SAE NO 


Fig. 361. Section showing the relations of the Devonian and other Paleozoic 
systems in the vicinity of Loudon, Tenn. =Cambrian; O=Ordovician; S=Si- 
lurian; D=Devonian; aw=age unknown. Length of section, about 7 miles. 
(Keith, U. S. Geol. Surv.) 
heavier subterranean water which tends to force them to the sur- 
face. In all cases it appears that there must be impervious beds 
above to prevent the escape upward of the oil and gas. 

The Devonian of central Tennessee is the horizon of black 
phosphates, which are of importance commercially. 


Foreign Devonian 

Europe. At the close of the Silurian there seem to have been 
more considerable geographic changes in Europe than in America, 
for the Devonian system there is more commonly unconformable 
on its base. During the progress of the period, Europe was pro- 
gressively submerged, for the Middle and Upper Devonian forma- 
tions are more widespread than the Lower (Fig. 362). 

In the British Isles the Devonian system has two phases. The 
first is found in the area which gave the system its name (Devon- 
shire). The system here is thick and of marine origin. Igneous rocks 
are associated with the sedimentary, and the system has valuable 
ore-bearing veins, as in Devon and Cornwall. 

The second phase of the Devonian is the Old Red Sandstone, 
widely distributed in Great Britain and Ireland and found at some 
points on the continent. Concerning the history of this sand- 
stone there has been much difference of opinion, but it is believed 
to have been deposited in a series of inland lakes or seas, the waters 
of which were fresh or brackish. Since species of marine fossils 
occur at some horizons, the sea had access to the basins at times. 
It is not improbable that some parts of this singular sandstone are 
of subaérial, rather than subaqueous, origin. The Old Red Sand- 
stone has some features like those of the Catskill formation of 
America. In the British Isles, the Old Red Sandstone has great 
thickness and includes much igneous rock. 

1 Columbia (Tenn.) folio, U.S, Geol. Surv. 


412 DEVONIAN PERIOD 


le = 


| i ™ 


a 


hh GG. 


A a i 
Mh it [ | 
A i ‘i | 
@ . : 


















































Fig. 362. Sketch map of Europe during the Devonian. The horizontal lines 
represent the Lower Devonian; the vertical lines mark the additional areas where 
the Middle Devonian occurs. (Z After De Lapparent.) 


In the Devonian of Germany much igneous rock is interbedded 
with the sedimentary. The igneous rock occurs in many separate 
beds, showing that there were many periods of igneous activity 
separated by intervals of quiet. In not a few places, especially 
where the sedimentary rocks have been invaded by igneous rocks, 
mineral veins have been developed, and from them large quantities 
of iron, tin, copper, and other metals have been obtained. 

The Devonian of Russia is made up of beds of arenaceous and 
calcareous rocks, the former containing fossils related to those of 
the Old Red Sandstone, the latter containing fossils of a marine 
fauna. The Lower Devonian appears to be wanting in much of 
Russia, and the Middle and Upper parts of the system are in most 
places unconformable on subjacent formations. 

Other continents. The Devonian system has wide distribu- 


tion in Siberia and China, and is known at many points in southern 


Asia. It occurs in North and South Africa, in New South Wales, 


chin. 


LIFE 413 


and Victoria and New Zealand, and the Lower Devonian especially 
has considerable development in South America. 


Climate 


Conclusive evidence of great diversity of climate, or of variations 
of climate during the period, are not at hand. The Old Red Sand- 
stone and the Catskill formation perhaps point to aridity, but this 
can hardly be affirmed. In formations thought to be Devonian, 
evidences of glaciation have been reported from South Africa,! 
but the evidence is perhaps not conclusive. 


LIFE 
The Marine Faunas 


At the beginning of the period shallow-water faunas were re- 
stricted to limited bodies of water about the continental borders. 
The life of these several bodies of water developed differently. The 
early Devonian life consisted of the expansions of these provincial 
faunas. When in the early Devonian the sea invaded the land from 
these different embayments, the advance from each carried its own 
somewhat peculiar fauna toward the interior. The faunas invaded 
the continent more or less simultaneously, but they reached the 
interior more or less successively. The following faunas have been 
recognized: (1) the Helderberg, (2) the Oriskany, (3) the Onondaga 
(Corniferous), (4) the Southern Hamilton, (5) the Northwestern 
Hamilton fauna, and (6) the late Devonian fauna. They reached 
the interior in the order named. As each in turn came in contact 
with the preceding fauna, there was a mingling of the two, resulting 
in the destruction of some species and the modification of others. 
A new, composite fauna developed from the survivors. 

Helderberg fauna. The Helderberg fauna seems to have 
developed from the late Silurian fauna in the embayment at the 
mouth of the St. Lawrence and on the border of the adjacent con- 
tinental shelf, and perhaps also on the border of southern Europe. 
It appears to have found its way into the Appalachian valley- 
trough, and thence to have spread westward and northward, but not 
beyond the eastern part of the great interior region. Perhaps it 
reached the interior also from embayments on the southern coast. 
The fauna had much in common with the contemporaneous fauna 
(Hercynian) of southern Europe, but both differed markedly from 

1 Schwarz, Jour, Geol., Vol, XIV, p, 683, and David, Q. J. G.S., Vol, XLIII. 


414 DEVONIAN PERIOD 


the early Devonian faunas of the northern latitudes of Europe and 
America. | 

The main features of the Helderberg fauna were great numbers 
of mollusks and brachiopods, an erratic tendency of the trilobites, 





Be Bete 


Fig. 363. HELDERBERGIAN Fossits: a, Polypora lilea (Hall), a fenestelloid 
bryozoan representative of a group which was of great importance later; b, Miche- 
linia lenticularis Hall, the earliest member of a genus of corals which became abun- 
dant inlater Devonian faunas; c, Lepocrinites gebhardii Con., one of the last represen- i 
tatives of the cystids. d-n, Brachiopods: d, Rensseleria @quiradiata (Con.), a 
representative of a genus characteristic of the Lower Devonian; e, Spirifer macro- 
pleurus (Con.), a species closely related to the Silurian species of the genus; f, : 
Strophonella punctulifera (Con.); g, Schizophoria multistriata (Hall); h, Uncinulus 
mutabilis (Hall), a representative of a genus which had its greatest development in 
the Helderbergian fauna; 7, Gypidula galeata (Dal.), one of the most characteristic 
species of the Lower Helderberg; j, Bilobites varicus (Con.), a type of orthid charac- ; 
teristic of the Silurian and Helderbergian; k, Eatonia medialis (Van.), a representa- ‘ 
tive of a genus most characteristic of the Lower Devonian; /, Rhipidomella oblata 
(Hall); m, Leptena rhomboidalis Wilck., a species which ranges from the Ordovician 
to the Mississippian; x, Airypina imbricata (Hall), a lingering Silurian type; 0, 
Actinopteria textilis (Hall), a winged pelecypod of a type which had great expansion 
in the Devonian; p, Plalyceras gibbosum Hall, a capulid gastropod; g, Dicranurus 
hamatus (Con.), a trilobite whose closest relative occurs in Barrande’s Etage G, in 
Bohemia; 7, Phacops logani Hall, a representative of a genus of trilobites which had 
its greatest development in the Devonian. | 





LIFE PP er 


a paucity of crinoids and corals, and a notable absence of jishes. 
Fig. 363 shows some of the characteristic forms. 

Oriskany fauna. The Oriskany fauna was a sand-loving fauna 
which followed the Helderberg into the interior apparently by a 
similar route. Its place of origin is not known with certainty, but 
its habitat was probably on the Atlantic coast. It was bound by 
many ties to the Helderberg fauna, but contained distinctive 
features, implying a partly separate origin. On the whole, this 
fauna was essentially an assemblage of well-fed mollusks and mol- 
luscoids, with but a sprinkling of other types. Brachiopods were, 
on the whole, the most distinctive forms. 





Fig. 364. OrisKANY Fossits. Brachiopods: a, Rensseleria ovoides (Eaton); a 
representative of a genus restricted to the Helderbergian and Oriskany (see Fig. 
363, d); b, Hipparionyx proximus Van., one of the most characteristic fossils of the 
arenaceous Oriskany beds; c, Camarotechia barrandei (Hall), one of the large 
rhynchonelloid shells of the Oriskany; d, Spirifer murchisoni Castel, and e, S. areno- 
sus (Con.), two of the most characteristic Oriskany species, the first occurring 
throughout the fauna, the second mainly in the fauna of the arenaceous beds; 
f, Stropheodonta magnifica Hall, a species which sometimes grew to be four or five 
inches across. The genus has its great expansion in the Devonian. The figures 
are much smaller than the fossils, the largest shells being 4 to 5 inches across. The 
large size of the Oriskany brachiopods may be appreciated by comparison with Fig. 
363, the brachiopods of which are reduced to the same extent as those of this Fig. 


Onondaga fauna. The Onondaga fauna was distinguished 
from the preceding by hosts of marine fishes of divergent types. 
From this time on fishes were abundant in the epicontinental waters 
of America and Europe, and doubtless ranged widely over the seas. 


a6 | DEVONIAN PERIOD 


A feature of the Onondaga formation consists of thin layers (‘‘bone- 
beds’) made up almost wholly of their plates (scales), teeth, spines, 
etc. Among the fish were (1) arthrodirans whose necks were so 
joined to their bodies as to give their heads vertical motion, a rare 
feature among fishes; (2) sharks of various types; and (3) ganoids 
with cartilaginous skeletons and bony scales, in contrast with the 
modern feleosts which have bony skeletons and membranous scales. 
These fishes seem to have been more fully clothed with spines and 
defensive armor than their descendants. Compared with existing 
species, they were doubtless heavy, clumsy, and sluggish. From 
the degree of development already attained, it may be inferred that 





Fig. 365. ONONDAGAN Fosstts: a, Zaphrentis ponderosa Hall, a medium- 
tized, simple horn coral; b, Nucleocrinus verneuili (Troost), a blastoid abundant in 
one layer of the Onondagan limestone in the Ohio Valley; c-h, brachiopods: c, 
Stropheodonta concava Hall; d and e, Productella spinulicosta Hall, an early represen- 
tative of a genus which became abundant in the Upper Devonian, and gave rise to 
the typical Productus of the Mississippian and Pennsylvanian faunas; f, Spirifer 
acuminatus (Con.), a characteristic Onondagan brachiopod; g and h, Crytina 
hamiltonensis Hall, two views of a species having a wide geographic distribution 
and a great geologic range in the Middle and Upper Devonian; 7, Tornoceras 
mithrax (Hall), the first goniatite in America. The goniatites are distinguished 
from earlier cephalopods by their lobed sutures; 7, Conocardiwm trigonale Hall, a 
dorsal view of a common Onondagan pelecypod; k, Platyceras dumosum Con., a 
capulid gastropod with large hollow spines; /, Odontocephalus egeria (Hall), a trilo- 
bite showing ornamentation of the border of the head and tail. 


LIFE 417 


their ancestors had been living for a long time in the region where 
they originated, probably somewhere in the north. 

Another significant feature of the Onondaga fauna is the pro- 
fusion of corals. From the rapids of the Ohio at Louisville, more 
than 200 species have been collected, embracing both the simple 
cup form (a, Fig. 365) and the compound type. Some of the cup 
corals attained a length of 18 inches and a diameter of 3, but the 
range in size was great. The reef-building habit attained greater 
development than in Silurian times, the reef at the rapids of the 
Ohio being the most famous example. Crinoids were rather few, 
but they do not appear to have lost their vitality, for they were 
abundant later. Large Brachiopods and cephalopods were plentiful. 
It will be remembered that in the primitive types of the cephalo- 
pods, the septa of the shells were plane or symmetrically curved, and 
that their juncture with the outer shell was a simple curve. In the 
Onondaga epoch, one form had septa which were bent abruptly, and 
suture lines which were lobed (7, Fig. 365). This was the first 
notable step in a remarkable series of crumplings of the septa which 
developed later. Gastropods similar to those of the earlier Devo- 
nian faunas were present, and the spines of the shells had now become 
pronounced in one group of them, perhaps signifying the necessity 
of defense against the abundant fishes and cephalopods. Pelecy- 
pods were abundant, many of them descended, no doubt, from 
Helderberg and Oriskany ancestors. Trilobites were present in 
more than half a hundred species, some of them being highly orna- 
mented. 

It seems clear that some of the species were descendants from 
the Helderberg and Oriskany faunas. Other prominent elements 
of the fauna, particularly the fish, cephalopods, and corals, seem, 
with equal clearness, to have come in from some other source. 
The striking features of the fauna seem to be explained by supposing 
that there was a generating tract to the north,' either on the Ameri- 
can or European continent, and that from this source migration 
into the interior sea of North America took place as the waters from 
the north extended themselves over the continent. As the result 
of the invasion, some part of the Oriskany fauna which already 
occupied the interior sea was driven out or destroyed, while the 
rest intermingled with the northern invaders. 


1 This conclusion is not universally accepted. See Schuchert, Bull. Geol. Soc. 
Amer., Vol. XX. 


418 DEVONIAN PERIOD 


Southern Hamilton fauna. At the beginning of the Hamilton 
epoch, there was a great influx of muddy material into the eastern 
part of the interior sea, while farther west the formation of limestone 
continued as before. At about this time, it appears that a fauna 
whose forbears lived in South America entered the interior sea, and, 
joining the resident Onondaga fauna, gave origin to the Southern 
Hamilton fauna. The transformation was not so radical as that 
which attended the invasion which gave rise to the Onondaga fauna, 
because the invaders were then the master type. 

Fishes were a conspicuous part of the new fauna. The arthro- 
dirans reached their climax, and some of the species were among 
the largest fish ever known. Some of them had an estimated length 
of 20 feet, and had strong mandibles 2 feet long (Fig. 366) which, 





Fig. 366. Diagrammatic front view of the dentition of -Dinichthys herzeri, 
Huron Shales, Delaware, O. (After Newberry.) 


in lieu of teeth, had cutting edges that closed, shears-like, after the 
fashion of the mandibles of turtles. The front part of the body was 
encased in heavy plates. Some of the fin-spines of sharks were a 
foot long. - In both groups of fish the devices of warfare make up 
nearly the whole record, and this doubtless implies the conditions 
in which the vertebrates lived. 

Polyps were affected adversely by the muddy waters. Crinoids 
were abundant locally, certain beds of limestone being composed 
largely of their remains. Brachiopods reached their climax at 
about this time. Among them, the spirifers attained their greatest 
extension of hinge-line (j, Fig. 367) a feature characteristic of the 
Hamilton epoch. The muddy bottoms favored mollusks. Gonta- 
tites increased in numbers and size (Fig. 367, 0), and pelecypods 
still more, the number of known species approaching’ 200. At this 
time appeared the first known barnacles of the modern sessile type. 
In losing its pedicel and in fixing itself immovably on other objects, 
it became degenerate, but it found a lowly place to which it has 


sate: 5, 
SES 


ie 2.1 
BA ATs tracker 





Fig. 367. REPRESENTATIVE HAMILTon Fossits: a, Fenesteila emaciata Hall, 
a type of bryozoan common in the Middle Devonian; b, Arthracantha punctobrachi- 
ata Williams, one of a genus of crinoids restricted to the Middle and Upper Devo- 
nian; ¢c, Eleutherocrinus cassedayi S. and Y., a peculiar, irregular blastoid; during life 
it probably rested upon one side on the sea bottom. d, Echinocaris punctata (Hall), 
a crustacean more highly organized than the trilobites. e-j, brachiopods: 
e, Tropidoleptus carinatus (Con.); f andi, Chonetes coronatus (Con.); g, Vitulina pustu- 
losa Hall; h, Rhipidomella vanuxemi Hall, a representative of the orthids, which 
had great development in the Devonian; 7, Spirifer pennatus (Atw.), one of the 
long-hinge-lined spirifers most conspicuous in the Middle and Upper Devonian; 
k, l, and m, pelecypods: k, Cypricardella bellistriatus (Con.); 1, Pterinea flabella 
(Con.); m, Paleoneile constricta (Con.); three pelecypods common in the Hamilton. 
n, Loxonema hamiltonie Hall, a gastropod common in this epoch; 0, Goniatites 
vanuxemi (Hall), a characteristic cephalopod of this fauna; », Phacops rana (Greene) 
the most common trilobite of the Hamilton, and representative of a genus which 
has its greatest expansion in the Devonian; qg, Crypheus boothi Greene, one of the 
last of the dalmanites. 


hung with wonderful persistence, not unlike the debased human 
‘class which it has come to typify. 

Northwestern Hamilton fauna. While the preceding fauna was 
developing in the eastern interior sea, another fauna was evolving 
on somewhat different lines in the northwestern sea which over- 
spread a large part of the northwestern interior (Fig. 358). Fora 
time this northwest sea was not in communication with the sea in 


420 DEVONIAN PERIOD 


which the Southern Hamilton fauna lived (Fig. 358), but the inter- 
vening barrier disappeared finally, and the northwestern fauna 
overran the territory already occupied by the Southern Hamilton 
fauna (Fig. 359). This northwestern fauna was closely allied to the 
Devonian fauna of eastern and central Europe. The southward 
extension of this great arm of the sea took place late in the period, 
for the strata bearing its peculiar life lie on pre-Devonian formations 
in Missouri, Iowa, and Minnesota, and overlie the Hamilton in the 
more eastern region. 

Later Devonian (Chemung) fauna. The commingling and 
conflict which attended the invasion of the eastern and southern 
interior sea by the European and Eurasian faunas may be regarded 
as the controlling event in the evolution of the Upper Devonian 
fauna. As in the case of the Onondaga invasion, the northern 
immigrants were the more virile, and gave character to the com- 
posite fauna that arose later from the extinction of the weaker 
species, and the adaptation of the survivors to one another. ‘There 
were three dominant factors in this development, (1) the resident 
Southern Hamilton species, (2) the invading European and Eurasian 
species, and (3) the shallow and rather turbid waters in which these 
species met and merged. ‘The last of these factors showed itself in a 
notable rarity of corals. The brachiopods best express the outcome 
of the commingling of resident and immigrant species. Among 
them, as in the whole fauna, there was an indigenous set of species 
developed from the preceding residents, and an exotic set derived 
from the immigrants and bearing North-European characters. The 
latter was the more conspicuous. Among the mollusks, however, 
the case was the reverse, and the majority seem to have been de- 
scendants of the resident bivalves. 

Devonian fauna in the Great Basin area. In the Great Basin 
region of the west, a large area seems to have been occupied con- 
tinuously by the sea from about the beginning of Middle Devonian 
time to the later portion of the Carboniferous period. It seems to 
have been measurably free from both the physical and the biological 
changes which gave such diversity to the eastern provinces. Its 
fauna had a slow, continuous evolution, favored, from time to time, 
it would appear, by accessions from the north, and perhaps from 
other sources as well. None of the distinctive South American 
forms appeared in it, nor any of the peculiar Helderberg or Oriskany 
species. It is inferred, therefore, that it was shut off from the 


LIFE 421 


eastern and southern interior throughout the whole Devonian period. 
On the other hand, a notable number of species were common to it 
and to the northwestern province. 


Life of Land Waters 


Certain Devonian formations, such as the ‘‘Old Red Sandstone’”’ 
and the Catskill formation, appear to be composed of deposits laid 
down in more or less local lodgment basins that were progressively 
filled by land-wash and fresh-water sediments. These basins appear 

to have been the home of a fresh- or brackish-water fauna, among 
which fishes, crustaceans, and ostracoderms were conspicuous. 
Perhaps the geological record presents no more suggestive combina- 
tion of ancient life. The type of the fauna was foreshadowed by the 
eurypterids and fishes, or fish-like forms of the late Silurian; but 
the record of that time is less perfect than that of the late Devonian. 

‘ The center of interest in this fauna is found in the ostracoderms 
(Figs. 368 and 369), a class of animals between arthropods and 





Fig. 368. Restoration of Cephclaspis, seen from the side. (After Patten.) 


vertebrates. Their chief interest lies in their suggestion that 
vertebrates sprang from arthropods. The ostracoderms bear ex- 
ternal resemblances, in the head and trunk, to trilobites and king- 
crabs, while some of them have caudal fins and fish-like bodies. 
They were formerly classed as fishes, but no vertebre have been 
found, or appendages or jaws of the vertebrate type. Ostracoderms 
probably formed the climax and almost the end of their own strange 
race, for they practically disappeared with this period. This is not 
surprising in view of the development of powerful fishes, for the 
ostracoderms were obviously not a masterful race. Besides being 
small, they were clumsy, and their mouth-parts were weak. They 
probably plowed the soft bottoms of the sluggish waters, half buried 
in the mud, above which little beside their peculiarly placed eyes 
and the backs of the plated bucklers were habitually exposed. 
Another class of strange organisms related to the fishes, but not 


422 | DEVONIAN PERIOD 


true fish, was represented by the singular little Palgospondylus 
(Fig. 370), which represents the vertebrate idea in great simplicity. 





Fig. 369. Reconstruction of the head and trunk of Tremataspis, seen from 
above. Natural size. (After Patten.) 





Fig. 370. Paleospondylus gunni, restored by Traquair; from the Old Red 
Sandstone, Caithness, Scotland. (After Dean.) 


It had a slender column of vertebrae, modified at one end into a 
head and finned at the other for a tail, without ribs, paired fins, or 
any suggestion of limbs. 

The fishes found in the supposed fresh-water deposits of the 
Devonian exceed in number and variety those found in contempo- 
raneous marine formations. Perhaps the strangest of them were 
the arthrodirans (Fig. 371), probably related to the ancestors of 
lung-fishes (Dipnot) which reached their climax at about this time. 
Ganoids were present, with many resemblances to amphibians, of 


LIFE 423. 


which they were, perhaps, the ancestors. Like lung-fishes, they 
appear to have been near their climax at this time, though they 
lived on till the Cretaceous. Sharks, now chiefly marine, seem to 
have lived in the open sea in the Devonian period, but their remains 





Fig. 371. A partial restoration of Coccosteus decipiens; from the Old Red Sand- 










BRON 


YY 
Owe 





are found also in the Old Red Sandstone and equivalent formations, 
so that they probably lived in fresh and brackish waters as well as 
in the ocean. 

Shells, probably of fresh-water mollusks, and closely resembling 
living genera have been found in association with land plants and 
fishes. 

Land Life 

Plants, snails, insects, myriapods, scorpions, and amphibians 
represent the known life of the land. 

The Devonian period covers much of the early development, 
though probably not the actual beginning of terrestrial plant life. 
It saw the origin of ferns, scouring rushes, lycopods, the seed- 
bearing relatives of the conifers, and probably the ‘‘seed-bearing”’ 
ferns... Devonian plants had, on the whole, little foliage, their 
leaves being spinoid and small. The presence of most of the 
fossil remains in fresh or brackish water or lowland deposits gives a 
suggestion of the habitats of the flora. It is inferred from the 


‘1 David White, Jour. Geol., Vol. XVII, 1909. Many of the statements of the 
following paragraphs are from this article. 


424 DEVONIAN PERIOD 


fossils that some of the plants were unable to stand alone, but 
sprawled about on the ground or clambered over other plants. Of 
the upland vegetation nothing is known. 

The Middle Devonian flora of Maine is so like a flora of Scotland, 
Belgium, and the Rhine provinces, as to indicate the probability of 
the migration of land plants between our continent and Europe, 
perhaps by way of a land bridge between America and Europe in 
the high latitudes. The Portage flora of New York is found also in 
Bohemia. The Upper Devonian flora was very similar from Penn- 
sylvania to southern Europe, and this flora has something in com- 
mon with that.of Australia. Devonian fossil woods show no rings 
indicative of seasons or long periods of drought. 

The types of Devonian plants were similar to those of the next 
period. The dominant forms were fern-like plants, some of which 
were seed-bearing, and the lower gymnosperms. ‘The forerunners 
of both lepidodendrons and sigillarias' were present before the close 
of the period. Angiosperms had not yet come into existence, so 
far as known. The forests were made up chiefly of (1) calamites 
(Equisetales) the gigantic ancestors of the horsetails, (2) lepidoden- 
drons, gigantic ancestors of the clubmosses, and (3) cordattes, all of 
which were better developed later. 

The record of the lower land plants is almost negative, except 
that, singularly enough, bacteria have been reported. The identi- 





Fig. 373. A, Platephemera antiqua, Sc., St. Johns, N. B. (After Scudder.) 
B, Xenoneura antiquorum, Sc. From St. Johns, N. B. (After Scudder.) 
fication of such simple forms in fossilized woody tissue of so ancient 
a period is remarkable, though the presence of bacteria is altogether 
probable in itself, for the record of plant life should have been more 
perfect than it is, had decay not been promoted by bacteria. 

The general aspect of the fern-like, seed-bearing plants was 

1 For classification, see p. 685. 


LIFE 425 


very like that of existing ferns. The larger number were herbaceous, 
but there were tree-forms not unlike tree-ferns in general appear- 
ance. ‘These plants were already far advanced in their evolu- 
tion, though little is known of their antecedents. They are gen- 
erally thought to have been the progenitors of cycads and of most 
or all other gymnosperms. In numbers, fern-like plants appear 
to have surpassed all others. 

Numerous wings and other fragments of insects have been 
found, chiefly near St. Johns, New Brunswick. Myriapods (thou- 
sand legged worms), arachnoids (spiders), and scorpions have been 
reported, and also terrestrial mollusks. 


CHAPTER XIX 
THE MISSISSIPPIAN (EARLY CARBONIFEROUS) PERIOD 


The time from the close of the Devonian period to the end of 
the Paleozoic era was formerly regarded as the Carboniferous period. 
But this interval is now divided into two or three divisions, each 
with the rank of a period. If three divisions are made (as here), 
the first is the Mississippian (Subcarboniferous, Lower Carbonifer- 
ous) period. It represents a time of widespread submergence of 
the North American continent, and was brought to a close by wide- 
spread emergence. The second, the Pennsylvanian (Carboniferous, 
Coal Measures, Upper Carboniferous) period represents a time 
when the area between the Appalachian Mountains and the tooth 
meridian maintained a halting attitude, being now slightly above 
sea-level and now slightly below it. West of the Great Plains, sub- 
mergence was rather general, as during the preceding period. The 
third division of the old Carboniferous period is the Permian, a 
time of notable crustal deformation, general aridity, and, during part 
of the period at least, low temperature. 


FORMATIONS AND PHYSICAL HISTORY 


The foliowing subdivisions of the Mississippian system are 
recognized in the regions indicated: 


Mississippi River States Pennsylvania 
4. Chester (or Kaskaskia) series (including Cypress 
sandstone below, and Chester beds above). 2. Mauch Chunk © 


3. St. Louis series (including Salem limestone below 

and St. Louis and St. Genevieve limestones 

above). 
2. Osage or Augusta (including the Burlington 

and Keokuk limestones, and Warsaw shale). 1. Pocono 
1. Kinderhook (or Chouteau) 


East of the Great Plains 


In the early part of the Mississippian period, coarse sediments 
(sands and gravels, now a part of the Pocono formation) were gather- 
ing along the western border of Appalachia, while in the central 
part of the Mississippi basin the sediments of this stage ( Kinder- 


426 


FORMATIONS AND PHYSICAL HISTORY 427 


hook) were partly calcareous. At the same time, the area of South- 
ern Michigan was a sort of bay or partly enclosed sea receiving sedi- 
ment from surrounding lands. Most of these formations are marine, 
but the Pocono has yielded fossils of land life. The formations of 
this stage are less widespread than those of later stages. 

In the second (Osage or Augusta) stage of the period, the sea of 
the interior was clearer, and the deposition of limestone was general. 
Submergence extended westward, probably to New Mexico on the 
one hand and to Montana on the other. The rich deposits of zinc 
ore (with some lead) in southwestern Missouri and eastern Kansas 
are chiefly in the Osage beds, though the metallic compounds were 
concentrated into ores at a later time. 

East of the Cincinnati arch, which was probably an island at this 
time, the deposition of clastic sediments continued. Those of 
eastern Ohio constitute a part of the Waverly series. Farther east, 
the accumulation of sand and gravel continued, or had been suc- 
ceeded by the deposition of the mud which constitutes the Mauch 
Chunk formation. The sediments of at least a part of this formation 
seem to have accumulated on land, rather than in the sea. In 
Maryland and elsewhere farther south, a formation of limestone 
(Greenbrier) lies between the Pocono and the Mauch Chunk. 

The St. Louis stage marks the time of maximum Mississippian 
submergence, so far as the western interior is concerned (Fig. 374). 
Limestone deposition continued in the Mississippi basin. It was 
at this time that the Bedford limestone! of Indiana (Salem or 
Spergen formation), famous as a building stone, was deposited. 
Much of this limestone, long mistaken for odlite, is made up of the 
shells of foraminifera. Many of the great limestone caves in Ken- 
tucky and southern Indiana are in the limestone of this epoch. In 
Michigan, beds containing salt (brine) and gypsum were being laid 
down, as at certain earlier stages in the period. 

In the northern part of the Appalachians, the Mauch Chunk 
shales were in process of deposition. Other names are applied to 
the contemporaneous deposits in the mountains farther south. 
Locally, deposits of this time contain both coal and iron ore. 

The Chester stage of the period was marked by more restricted 
waters and more varied sedimentation. The deposits of this stage 
resemble in a general way those of the Kinderhook stage. Those 


1This name as applied to this limestone, is a trade name. As a geological 
term, Bedford is applied to a member of the Waverly series farther east. 


428 MISSISSIPPIAN PERIOD 





Fig. 374. Map showing the areas, in black, where the Mississippian system 
appears at surface. The map also shows where the Mississippian system is thought 
to exist, though buried (the lined areas), and the area from which it is thought to 
have been removed by erosion (the dotted areas). By inference, also, the map shows 
the relations of land and water during the Mississippian period. 





FORMATIONS AND PHYSICAL HISTORY 429 


were made while the sea was advancing on the land, these while it 
was retreating. Both are more restricted in their distribution than 
the beds of the intermediate epochs. In Illinois, the Chester sand- 
stone bears oil locally.! 

In summation it may be said that the Mississippian beds are 
largely clastic east of the Cincinnati arch, and largely calcareous 
west of it. It should be added also that the history of the Missis- 
sippi basin in this period is less simple than the preceding sketch 
might seem to imply, since there are several unconformities in the 
system, implying repeated emergencies of considerable areas. The 
extent of these unconformities has not been determined. 

In Nova Scotia, the system rests, locally, on much older forma- 
tions, and contains beds of red sandstone and gypsum. 


In the Great Plains and West of Them 


The Mississippian system is known in Oklahoma and South 
Dakota, where deformation and erosion have brought the strata to 
the surface (Fig. 374). Farther west the distribution of the system 
shows that the present mountain region, as far west as the 117th 
meridian, was mostly submerged, though there were perhaps numer- 
ous islands. North of the United States, also, marine conditions 
prevailed widely. Much of the system in the west is limestone, 
though clastic formations are not wanting. ‘The system is exposed 
about many of the mountains, and over considerable areas in 
Arizona and perhaps in New Mexico. It rests on the Ordovician 
in many places, and locally overlaps all earlier Paleozoic systems, 
lying on the Proterozoic. In parts of Colorado (Leadville) the Mis- 
sissippian limestone and dolomite constitute one of the richest ore 
horizons of the state. In many parts of the west the Mississippian 
system is unconformable beneath the Pennsylvanian.’ 

Igneous activity. According to present interpretations, there 
was great igneous activity in the west during this period. The 
area affected by vulcanism at this time, or soon after, extended 
from Alaska on the north to California on the south.* Dikes 
affect the system of Southern Illinois and adjacent parts of Ken- 
tucky, but the date of their intrusion is not known. 

1 Bain, Econ. Geol., Vol. III, and Bull. 2, Ill. Geol. Surv. 

2 The Mississippian is not differentiated from the Pennsylvanian on the maps 
of most of the western folios of the U. S. Geol. Surv., though the two are differ- 
entiated in the texts especially in the later folios, 

3 Dawson, Can. Geol, Surv., 1886, p. 85. 


430 MISSISSIPPIAN PERIOD 


General Considerations 


Thickness and outcrops. In keeping with the variations in 
the sediments, the thickness of the Mississippian system varies 
greatly. In Pennsylvania, there is a thickness of 1,400 feet of 
sandstone (Pocono), with 3,000 feet of shale (Mauch Chunk) above 
it; but so rapidly do the formations thin westward, that in the 
western part of the same state the equivalent formations have a 
thickness of only 300 to 600 feet. In the region of the Mississippi 
it reaches a maximum thickness of about 1,500 feet. In Oklahoma, 
the thickness is about 1,800 feet, in the Black Hills 275 to 525 feet, 
in Colorado (Crested Butte region) 400-525 feet, and in northern 
Arizona (Grand Canyon of the Colorado), 1,800 feet. 

Close of the period. At the close of the period, the eastern 
interior sea was contracted to narrow limits if not obliterated. 
Great changes took place in the western half of the continent too, for 
there is a widespread unconformity above the Mississippian system. 
In parts of the west, however, so far as now known, marine conditions 
prevailed uninterruptedly from the early Mississippian period to the 
later part of the Pennsylvanian. 

This great unconformity, and the great changes in life which 
accompanied the emergence which it records, is the basis for 
regarding the Mississippian a distinct period. 


Lower Carboniferous of Other Continents + 


In western Europe, two great series, or systems, are included 
under the Carboniferous, (1) the Lower Carboniferous, chiefly of 
marine origin, and (2) the Coal Measures or Carboniferous proper, 























Fig. 375. Composite diagrammatic section, showing the unconformity between 
the Mississippian and Pennsylvanian systems in Iowa. (Keyes, Ia. Geol. Surv.) 


' The term Lower Carboniferous is here used, instead of Mississippian, because 
it is the term in common use in Europe, 


FORMATIONS AND PHYSICAL HISTORY 431 


deposited partly in lagoons, marshes, and lakes, and partly in the 
sea. These systems correspond, in a general way, to the Missis- 
sippian and Pennsylvanian of North America. In the southern 
part of the continent the Lower and Upper Carboniferous forma- 
tions are like the Mississippian and Pennsylvanian of western 
North America, in that both are chiefly marine. In eastern Europe 
the Lower Carboniferous is partly non-marine and coal-bearing, 
while the Upper Carboniferous is largely marine. 

The Lower Carboniferous of western Europe is largely of lime- 
stone, which in Great Britain has received the name of ‘‘mountain 











































































































Fig. 376. Map showing the relations of land and water in Europe in the early 
Carboniferous period. The shaded parts represent areas of marine deposition. 
(After DeLapparent.) 


limestone.’ East of the Rhine the Lower Carboniferous limestone 
is replaced by shale, sandstone, and even conglomerate, collectively 
known as the Culm. This phase of the system contains coal in 
some places. 


432 MISSISSIPPIAN PERIOD 


The Lower Carboniferous of some parts of Great Britain and 
western Europe contains much volcanic rock. Some of the erup- 
tions were probably submarine, and some subaérial. 

The close of the early Carboniferous period was marked, in 
Europe, by widespread withdrawal of the sea from the area of the 
continent which it had covered. There were also some mountain- 
forming movements (folding), as in the Vosges Mountains, in east- 
ern France, and elsewhere. The development of the Ural Moun- 
tains appears to have begun at about the same time. These changes 
shifted the areas of sedimentation notably. 

In other continents, where geological work is less advanced, the 
Lower and Upper Carboniferous have not always been separated 
carefully, but the lower system exists in all of them. 


Climate and Duration 


Most of the data at hand indicate the absence of great diversity 
of climate during the period, and suggest that it was genial. The 
salt and gypsum in Montana, Michigan, Nova Scotia, and western 
Australia, imply aridity, but it is not clear that aridity was general. 
Certain conglomerate formations (in the Culm) of western Europe 
have been thought to indicate glaciation, but the evidence does not 
seem to warrant this conclusion. Recently, phenomena which have 
been interpreted to imply floating ice have been reported from 
Oklahoma.! The duration of the period probably was not less than 
the average duration of the Paleozoic periods. 


LIFE 


Marine faunas. Just as there was no great stratigraphic break 
between the Devonian and Mississippian systems in the American 
continent, so there was no radical break in the succession of life. 

Conspicuous elements of the Kinderhook fauna were (1) the 
beginnings of the great deployment of the crinoids, which reached 
their climax later in the period; (2) brachiopods, which were transi- 
tional between Devonian and Later Mississippian types, the genus 
Productus being conspicuous (Fig. 377, d. e.); and (3) abundant mol- 
lusks, pelecypods (i, 7, Fig. 377) being most numerous. Trilobites 
were few and small. Their high stage of ornamentation had passed, 
and the day of their disappearance was drawing near. Fishes, 
especially sharks, were abundant. 

1 Taff. Bull. Geol. Soc. Am. Vol. xx. p. 701. 


LIFE 433 





“4M 
* “Fig. 377. Krnperuook Fossits: a, Leptopora placenta (White), a compound 
coral. 6, Actinocrinus senectus M. and G., a distinctively Mississippian crinoid; 
c, Dichocrinus inornatus W. and Sp., one of the earliest crinoids with only two basal 
plates. d-h, brachiopods: d, Spirifer biplicatus Hall, a species retaining an elon- 
gate hinge line characteristic of the Devonian; e, Spirifer marionensis Shum.; f, 
Productella pyxidata Hall, a genus which had its greatest development in the late 
Devonian; g, Paraphorynchus striatocostatus (M. and W.), characteristic of Lower 
Kinderhook horizons of Iowa, Missouri, and Illinois; h, Productus arcuatus Hall, a 
genus developed from Productella, and characteristic of the Mississippian and later 
Paleozoic periods; 7, Grammysia hannibalensis (Shum.), a pelecypod; 7, Pernopecten 
cooperensis (Shum.), a pelecypod characteristic of certain of the higher Kinderhook 
horizons; k, Platyostoma broadheadi S. A. M., a capulid gastropod; 1, Macrocheilus 
blairi (M. and G.); m, Prodromites gorbyi (S. A. M.),a widely distributed cephalopod 
and the earliest form showing secondary lobing of the sutures; 1, Muensteroceras 
owent (Hall), abundant in the famous Kinderhook goniatite bed at Rockford, Ind.; 
0, Préetus ellipticus M. and W. Trilobites were few in the Kinderhook, and this 
one illustrates their characteristic lack of ornamentation; p, tooth of Cladodus 
springeri St. J. and W., a shark; g, a spine of Acondylacanthus gracilis St. J. and W. 


_ The physical conditions of the Osage epoch furnish the key to 
the character of the Osage fauna. The extended shallow, clear sea 





434 MISSISSIPPIAN PERIOD 


was a favorable field for the evolution of the varied assemblage of 
forms that had come together in preceding epochs under less favor- 
able conditions. ‘There is evidence also of rather free migratory 
communication with the Eurasian continent, since many species 
were common to America and Europe. 

No single group so well characterizes the Osage fauna and ex- 
presses its dependence on physical conditions as the crinoids, whose 
abundance and diversity were climacteric (Fig. 378). Their rapid 








Fig. 378. OsSAGE EcHINODERMS: 4-d, crinoids; a, Barycrinus hovevi Hall; 6, 
Eretmocrinus remibrachiatus (Hall), having spatulate arms; c, Actinocrinus lobatus 
Hall, shows highly ornamented plates; d, Forbesiocrinus wortheni Hall, a flexible 
crinoid; e, a blastoid, Oligoporus mutatus Keyes. 


decline after this epoch is one of the most remarkable incidents in 
the life history of the invertebrates. In the day of their glory, the 
crinoids were most prolific, as indicated by the fact that a single 
genus (Batocrinus), had more than a hundred species. Their orna- 
mentation was notable, and as in the case of the trilobites, preceded 
their decline. The repetition of this phenomenon at different times 
and in different groups of organisms is worthy of notice, though its 
meaning is not altogether clear. Crinoids made large contributions 


LIFE 435 


to the limestone of the period. Other echinoderms were not very 
abundant. 

It is a matter of surprise that corals were so few, in view of the 
favorable physical conditions. Their paucity probably is to be 
explained by unfavorable organic conditions or relations, such as 
unrecorded enemies, or more successful rivals. Brachiopods (Fig. 
379) were abundant, and some of their species ranged to the eastern 





g n i 


Fig. 379. OsAGE Fossits: a, Zaphrentis centralis E. and H., the most char- 
acteristic coral of the Osage. 0-7, brachiopods: 6, Spirifer suborbicularis Hall; a 
closely allied species occurs in Europe. c, Athvris lamellosa L’Eveille, a species 
common to America and Europe; d, Spirifer logani Hall, the American representa- 
tive of Spirifer striatus of the European Mountain limestone; e, Productus burling- 
tonensis Hall, a species abundant in the Lower Osage; f, Leptena rhomboidalis Wilck, 
a species which persisted from the Ordovician to the Osage; g, Rhipidomella burling- 
tonensis (Hall); h, Reticularia pseudolineata (Hall), a spire-bearing brachiopod 
closely allied to species in the European Mountain limestone; 7, Schizophoria 
swallovi Hall, one of the last of the orthids. 


continents. Mollusks were very subordinate. There were a few 
lingering ¢rilobites, an abundance of bryozoans, some supposed 
sponges, and doubtless many forms not readily fossilized. Marine 
plants left but an obscure record. 

The Waverly fauna, east of the Cincinnati axis, was more provin- 
cial than the Kinderhook and Osage faunas. It was the direct 
descendant of Devonian faunas that occupied the same ground, and 
had changed but slowly. It was modified by some immigration of 


436 MISSISSIPPIAN PERIOD 


Kinderhook and Osage types, and took on slowly a Mississippian 
aspect, while retaining many Devonian characteristics. Its most 
prominent members were the pelecypods, as might have been antici- 
pated from the silty conditions. 

The Great Basin fauna of the first half of the period records a 
gradual evolution of the Devonian fauna of the same region, with 
perhaps the addition of a few immigrants from the west. After 
the Osage epoch, the Basin fauna united with the Osage fauna of 
the interior, and this union had an important effect on the later 
Mississippian faunas of the interior. 

Previous to the union, the salient features of the Great Basin 
fauna were the (1) rarity of crinoids; (2) among brachiopods the 
absence of spirifers, so characteristic of the Osage fauna, and the 
presence of the genus Productus, closely allied to species of the Osage 
fauna and probably developed by parallel evolution; (3) the pre- 
ponderance of pelecypods over brachiopods; (4) the abundance of 
gastropods, among which were air-breathers, the oldest aquatic 

pulmonates known; and (5) plentiful cor- 
als, the horn-shaped type predominating. 
Cephalopods and trilobites were few, and 
no fishes have been reported. Unless this 
is due to the imperfection of the record or 
of present investigation, it adds much to 
the evidence of the distinctness of the pro- 
vince, for fish abounded in the eastern sea. 
The barrier which separated the Great 
Basin and the Kinderhook-Osage seas 
é appears to have been an elongated insu- 
Fig. 380. Upper Mis-_ lar tract lying between the Rocky Moun- 
SISSIPPIAN ECHINODERMS: tains and the Great Basin. The yielding 
ita ne? ee of this barrier about the close of the Osage 
which lost its stem and epoch, by erosion or submergence, permit- 
became a free swimming ted the singular semi-Devonian, semi- 
creature, at least in its See eA y i 
adult condition; 6, Acro- Mississippian fauna of the west to invade 
crinus amphora W.andSp., the greater eastern sea. The late Missis- 
a. specialized camerate cri- sip pian (St. Louis) faunas of the interior 
noid with a large number ; . 
of supplementary plates include (1) the culmination of the cosmo- 
introduced between the  politan evolution of the marine life of the 
basal and radials; ¢, Pen- Wrississippian period on the North Ameri- 


tremites robustus Lyon, a ‘ iS Soa ; 
blastoid. can continent, and (2) the initiation of its 





LIFE 437 
decline. The most distinctive feature was the commingling of the 
Great Basin and the Osage faunas. It introduced into the main 
Mississippian sea what seemed to be a retrograde change, for 
species of Devonian aspect that still lived in the isolated Great 
Basin province and elsewhere, migrated eastward, and their relics 
are found with species whose evolution had reached an advanced 
Mississippian phase. 

Crinoids were less plentiful than in the Osage fauna, and notably 
changed (Fig. 380). Of one group which had upwards of 300 species 
in the Osage fauna, less than 25 species are known in the later faunas, 
and among the 25, no Osage species is found. Other groups of 


Bs 
ec SASL eri ogt 8 


7) 


ie Ee 
yi | Baas 





Fig. 381. CHARACTERISTIC UppeR MIssIssIpPIAN Fossits: a, Endothyra 
baileyi Hall, a small foraminifer, much enlarged, abundant in the Bedford limestone 
of Indiana, and often mistaken, in the past, for an odlitic concretion; b, Archimedes 
swallovanus (Hall), a bryozoan having a peculiar screw-like axis for the support of 
the colony. c-h, brachiopods: c, Spiriferina spinosa (N. and P.), a genus which 
developed from Spirifer, and has its greatest development in the late Mississippian 
and Pennsylvanian; d, Seminula subquadrata (Hall), a species closely related to 
Pennsylvanian types; e, Spirifer increbescens Hall, a species characteristic of the 
later Genevieve faunas; f, Eumetria marcyi (Shum.), a representative of a genus 
abundant in the Genevieve faunas. It was present in the Kinderhook, but has not 
been found between the Kinderhook and the closing stages of the Osage; g, Produc- 
tus fasciculatus McCh.; h, P. marginicinctus Prout; 7 andj, pelecypods: 7, Schizo- 
dus chesterensis M. and W.; 7, Conocardium prattenanum Hall; k-m, gastropods: 
k, Bellerophon sublevis Hall; 1, Pleurotomaria nodulostriata Hall; m, Eotrocus con 
cavus Hall. mn and o, cephalopods: x, Orthoceras annulato-costatum M. and W., 
one of the ancient type of straight cephalopods, occasional species of which per- 
sisted to the end of the Paleozoic; 0, Goniatites kentuckiensis S. A. M. 


438 MISSISSIPPIAN PERIOD 


crinoids, however, did not show so remarkable a decline, and newand 
curious forms appeared. Blastoids had their climax here so far as 
numbers of individuals are concerned, although there was greater 
diversity in the Osage fauna. A swift decline seems to have fol- 
lowed this climax, and the beautiful forms disappeared for reasons 
quite unknown. 

Polyps seem to have profited by the decline of the crinoids, or 
for other reasons, for they were more numerous than in the Osage 
fauna. The simple horn-shaped forms were the most common. 
Bryozoans made a new departure in their mode of support. The 
delicate branches of their colonies could not extend themselves 
indefinitely without special means of support. As one mode of 
securing this support, the genus Archimedes (Fig. 381, 6), which made 
its first appearance in the Osage, secreted an axis with a spiral 
flange upon which the colony spread itself, producing a unique form 
resembling slightly Archimedes’ screw. Archimedes became so 
abundant in the Kaskaskia epoch that a part of the series is known 
as the Archimedes limestone, because of the great abundance of 
fossils of this genus. 

A notable change took place in the brachiopods (Fig. 381), 
though Productus (g and h) continued to be abundant and charac- 
teristic. An odd feature was the small size of the brachiopods in the 
Bedford limestone of Indiana. The associated fossils of other kinds 
also were dwarfed, implying pauperizing conditions of some sort, 
for the species seem to be identical with those that grew larger else- 
where. It is not improbable that this limestone was deposited in a 
partially isolated body of water that was so highly charged with 
lime and other salts as to be somewhat unfavorable to life. A 
similar dwarfed fauna is recorded from Idaho. 

Among mollusks, pelecypods (Fig. 381, 7,7) were rather abundant, 
and some of them still had a Devonian aspect. Those in the Indi- 
ana foraminiferal limestone were small, like the brachiopods. 
Gastropods were more diversified than in the Osage fauna, and some 
Devonian genera which apparently had been absent from the Osage, 
reappeared. Sharks (Fig. 382) were important and other fish were 
present. 

The most striking peculiarity of the fauna resulted from the 
invasion of the more conservative fauna of Devonian aspect from 
the sea of the Great Basin, and perhaps from a similar incursion of 
lingering forms from the Waverly gulf on the east. The remarkable 


LIFE 430 


thing is that these should have succeeded, so far as they did, in 
impressing themselves on the composite result, and in giving tone 
to the whole. It is more natural to expect an antiquated fauna to 
be overwhelmed by a younger and more progressive one. 





Fig. 382. Cladoselache fyleri Newb. Restoration by Dean. About 1/5 
natural size. From Cleveland Shales, Ohio. 


With the close of the Mississippian period, the chief center of 
life interest passes from the sea to the land, first to the vegetation 
of the Coal period, and then to land vertebrates. The history 
of the marine invertebrates will hereafter be followed with less full- 
ness. With the introduction of fishes it had reached its great 
adjustments, and its further history bears a close likeness to the 
struggles and adaptations of the history already sketched. 

Evolution of fishes. Many of the ancient invertebrates were 
fixed, and their migrations were confined to the early stages of their 
lives; but fishes were rovers. While restrained by conditions of food, 
temperature, etc., they were relatively independent of local condi- 
tions. They appear to have invaded effectually the open sea for 
the first time in the Devonian period, though at that time, marine 
fishes seem to have been fewer than those of inland waters. But by 
the middle of the Mississippian period, marine fishes were in un- 
questioned supremacy, while the fresh-water forms had declined 
notably, so far as the record shows. In the seas, the supremacy 
of the sharks was almost uncontested. They were more abundant, 


440 MISSISSIPPIAN PERIOD 


apparently, than in any later period. Some 600 species are known, 
more than half of them from North America. The fossils are chiefly 
teeth, spines, and dermal ossicles. Three-fourths of the species had 
crushing or pavement teeth, adapted to breaking the shells of 
mollusks and crustaceans, and the trituration of seaweeds. The 
arthrodirans and lung fishes had declined, as compared with the 
Devonian period. Of fishes frequenting inland and coastal waters, 
probably the culminating type was of the order to which the modern 
garpike belongs. The curious tribe of ostracoderms (p. 686). had 
nearly or quite disappeared. ) 


Land Life 


The record of land life is poor, but enough fossil plants have 
been found to show that the plant life of the early Mississippian 
land was little more than an expansion of that of the preceding 
period. There were, however, notable changes in detail. The geo- 
graphic diversity of the Mississippian floras was somewhat greater 
than that of the Devonian. The mid-Mississippian flora is 
thought by White ! to have had its origin on the islands of western 
Europe, and to have spread thence to Siberia and southward, even 
to South Africa and Australia; but by what route is not known. 
Seventy-five per cent of the species of a Mississippian flora of Argen- 
tina are identical with European species, a fact which suggests 
strongly a land bridge between South America and the continents 
just named. 

The flora of the closing stages of the period indicates adverse 
conditions of life, and prepares the way for the great floral changes 
which followed. From this stage comes the earliest wood which 
shows rings. 

The most interesting suggestion of advance in land life is found 
in the footprints of a supposed amphibian from the Mauch Chunk 
shale of Pennsylvania. They imply a stride of about thirteen 
inches, and a breadth between outer toes of eight inches. Nearly 
complete specimens of amphibia (labyrinthodonts) have been found 
in the Lower Carboniferous of Scotland. 

Probably insects and their allies lived, but their fossils have not 
been found. 

1 Jour. of Geol., Vol. XVII, 1909. 


CHAPTER XX 
THE PENNSYLVANIAN (UPPER CARBONIFEROUS) PERIOD 


FORMATIONS AND PHYSICAL HISTORY 


This system includes the Pottsville conglomerate (Millstone grit) 
below, and the Coal Measures above. Its most distinctive feature, 
so far as North America is concerned, is its coal. 


The Pottsville Conglomerate (Millstone Grit) 


The lowest formation of the system in the Appalachian region is 
sandstone or conglomerate, having different names in different 
regions. From its conglomeratic phase in the east, it grades into 
sandstone in the interior. It has not been recognized in the western 
part of America. Over wide areas it is unconformable on the Mis- 
sissippian system, as already noted. Locally as in parts of Illinois, 
the formation is oil-bearing. At various points in the east it con- 
tains thin beds of coal, and in the southern Appalachians, some 
thicker beds. 

The formation varies in thickness from a maximum of some 
1,500 feet in the Appalachians, to less than 100 feet in some parts of 
western Pennsylvania. It is so firmly indurated that the outcrops 
of its tilted beds have become ridges in many places. 


The Coal Measures 


Above the Pottsville conglomerate and its equivalents in the 
central and eastern parts of the continent, lie the formations known 
as the Coal Measures. ‘They consist of a succession of alternating 
beds of shale, sandstone, conglomerate, limestone, coal, and iron ore. 
The succession differs greatly in different regions, but shale perhaps 
recurs more frequently than other sorts of rock, and in thicker beds. 
Both the coal and some of the iron ore are in layers interstratified 
with the other members of the series, and are to be looked upon as 
strata of rock. Important as the coal and iron ore are from an 
economic point of view, they make up but a small part of the Coal 
Measures. There are many beds of coal in some regions, and some 


441 


442 PENNSYLVANIAN PERIOD 





Fig. 383. Map showing the areas where the Pennsylvanian system appears at 
the surface in North America. The map also shows, as in preceding similar cases, 
the areas where the Pennsylvanian system is thought to exist though buried (lined 
areas); the areas where it is thought once to have existed, but to have been removed 
by erosion (dotted); and by implication the relations of land and sea during the 


Pennsylvanjan period. 


FORMATIONS AND PHYSICAL HISTORY 443 


of them have great thickness (40 to 50 feet); yet the proportion of 
coal in the Coal Measures is rarely so much as 1:40, and that of iron 
ore is much less. ‘The classification of the Pennsylvanian system of 
the east now in common use is as follows?! 

4. Monongahela 

3. Conemaugh 

2. Allegheny 

1. Pottsville 

A twofold division is common farther west. Thus in Iowa the 
lower division is called the Des Moines, and the upper, the Mis- 
sourian. 

Productive coal-fields. The Pennsylvanian system does not 
contain coal in workable quantity everywhere, though coal is widely 
distributed as far west as the 
96th or 97th meridian in Okla- a &Fipla . 
homa, and nearly to the rooth | 
meridian in Texas. The pro- 
ductive coal areas of the system 
in North America are six in xs 
number, as follows:? 

Tee esuniracite. field; of 1... ——._$ —_- —-— pee eG 
eastern Pennsylvania, with an Fig. 384. Map showing the areas of 
area of 484 square miles. It anthracite coal in Pennsylvania. 
includes several elongate, nearly parallel, synclinal basins (Figs. 384 
and 385). From the associated anticlines, and from the neigh- 


Pennsylvanian 


\ 
0 
INA 








Fig. 385. Section across Panther Creek basin in the anthracite region of 
Pennsylvania, showing the structure and the coal beds (black). (Stoek, U. S. 
Geol. Surv.) 
boring shallower synclines, the coal beds have been worn away. 
The strata of this field may once have been continuous with those 
of the next. 

(2) The Appalachian field, which extends from Pennsylvania to 
Alabama (Fig. 386), has an area of about 70,000 square miles, of 


1 Prosser, Am. Jour. Sci., 4th series, Vol. XI, p. 191, 1901. 
2 22d Ann, Rept., U.S. Geol. Sury., Pt, II, p. 15, 


444 PENNSYLVANIAN PERIOD 


ix 


OS ee ep of me eg 


30 $00 Scale 3 Miles 





Fig. 386. Map showing the known distribution of coal in the United States. 
The black areas are the areas within which there is coal of the Pennsylvanian system 
(anthracite and bituminous). The areas marked by dots in Virginia and North 
Carolina represent Triassic (bituminous) coal. Those with vertical (lignite) and 
horizontal (anthracite, bituminous, and lignitic-bituminous) lines represent coal of 
the Cretaceous (Laramie) system, and those with diagonal (lignite) and crossed 
(bituminous and lignitic-bituminous) lines represent coal fields of Tertiary age. 
Some of the fields, as those of Washington and California, appear very small on this 
map. The Cretaceous and Tertiary areas include only those where there is known 
to be workable coal. (U.S. Geol. Surv.) 


which about 75 per cent contains workable coal. The western edge 
of the sharply folded Appalachian belt is the eastern edge of the 
Appalachian coal-field. With few exceptions, the strata of this 
field are horizontal, or gently undulating. 

(3) The Northern Interior field, confined to the southern penin- 
sula of Michigan, covers an area of about 11,000 square miles. The 
strata of this field dip gently toward its center. 

(4) The Eastern Interior field, centering in Illinois, covers an 
area of about 58,ooo square miles (Fig. 386), and about 55 per - 
cent of it is productive. This field is set off from the Appalachian 
field on the east, and from the Western Interior field on the west, by 
broad low anticlines from which the Coal Measures, if ever present, 
have been eroded. 

(5) Lhe Western Interior and Southwestern fields (lowa to Texas) 
covers an area of about 94,000 square miles. On the west this field 


FORMATIONS AND PHYSICAL HISTORY A4S 


is limited by the overlap of younger for- ( Feet 
mations. Except in Arkansas and Okla- | 
homa, where the strata are folded, the 
Coal Measures of this area are nearly 
horizontal. 

(6) The Nova Scotia-New Brunswick 
coal-field, on either side of the Bay of 
Fundy, contains an area of about 18,000 
square miles. The coal is bituminous, of 
good quality. 

Non-productive areas. In the vicinity 
of Narragansett Bay, the Carboniferous 
system has great thickness, and locally 
rests on beds of Cambrian age. Coal 
occurs here, but it is too highly anthracitic a 
(or graphitic) to burn readily. The beds 
are much deformed and are associated 
with igneous rocks. Carboniferous rocks 
occur at other points in New England, 
where they are partly igneous (Fig. 389) 
or meta-igneous, and partly meta-sedi- 
mentary. 

West of the Great Plains. The system 
is widespread west of the Great Plains, 
and probably underlies the Plains them- 
selves. With rare exceptions, the western 
beds are coal-less, the abundant coal of 
that region belonging to later systems. 
The coal-less phase of the system, the 
whole earth considered; is far more wide- 
spread than the coal-bearing. 

In some parts of the west, the Car- 
boniferous system includes formations 
which resemble the ‘‘Red Beds” of the 
next (Permian) system. This is the case 
in the southern part of the Rocky Moun- 
tain region, and in the plains adjacent, and 
here the separation of the Pennsylvanian 
system from the Permian is not very dis- : 
tinct, or has not been carefully worked out, 0 | 





1500 


The black 





Picea 


1400 





The Pottsville portion 


Monongahela Series 
the Allegheny portion from Armstrong Co. (White [David] and Campbell); 





1300 f 


(U.S. Geol. Surv.) 


a 


1200 


1100 


900 & 


Conemaugh Series 
‘ 


800 





700 & 
600 F 
500 B 


400 ae 


Co. (I. C. White); 
the Conemaugh portion from Fayette Co. (I. C. White); the Monongahela portion from Fayette Co. (Stevenson). 


Allegheny Series 


300 F 


Composite section of the Pennsylvanian system of Pa., compiled from various sources. 
the checked pattern limestone, the dots sandstone, and the broken lines shale. 


Pottsville Series 


Fig. 387. 
of the section is from Mercer 


bands represent coal, 


446 PENNSYLVANIAN PERIOD 


The Carboniferous system of the west includes 
all sorts of sedimentary rocks, among which are 
considerable thicknesses of limestone. They are 
exposed at many points (Fig. 383) and their exist- 
ence over wide areas where they are now covered 








Fig. 388. Section showing the position and relations of 
the Carboniferous section near Estillville, Ky. C=Carboni- 
ferous (including Mississippian); D=Devonian; S=Silurian. 
Length of section about 16 miles. (Campbell, U.S. Geol. Surv.) 
by later deposits is certain. The system is, how- 
ever, not continuous. Numerous islands of older 
rock probably maintained themselves throughout 
the period, and a large area of land existed through- 





ea Oe 


Fig. 389. Section in northwestern Massachusetts, showing 
the position and relations of the Carboniferous system. Cw= 
igneous rock, Carboniferous; Sc (Conway schist) and Sg 
(Goshen schist) are Silurian formations; Of (Hawley schist), 
Os (Savoy schist), and Och (Chester amphibolite) are probably 
Ordovician, though classed with the Silurian in the Hawley 
folio. (Emerson, U.S. Geol. Surv.) 





out the Paleozoic era in western Nevada (west of 
long. 117°), and had an unknown extension north 
and south. 

Figs. 390 to 392 show the positions and rela- 


tions of the Mississippian and Pennsylvanian sys- <(H} 
tems at various points in the west. The sections E¢ 
are from regions where the strata have been much z % 


disturbed by folding, faulting, and the intrusion 
of igneous rock. 

North of the United States, Carboniferous 
strata (largely Mississippian) outcrop on the west 
side of the northward continuation of the Great 
Plains. These strata are probably continuous 
southward with the contemporaneous formations 
of the United States. Strata of the same age are 
found on both sides of the Gold Range of British 





Rs 
Ai! 


oe, 
Sfmt 


4 
~ 


oe 
ARI 
2 


—_—/ 


ystem (as well as others) in the Yellowstone Park, 


fay 4 


Jurassic, K = Cretaceous. 


Carboniferous, J 
(Hague, Iddings, and Weed, U. S. Geol. Surv.) 


C= 


Devonian, 


gth of section about 17 miles. 


Ordovician and Silurian, D= 


Cambrian, S$ 


Section showing position and relations of the Carboniferous s 


Fig. 390. 
Archean, € 
Neocene, and anp=igneous rock. Len 


AR 
N 


FORMATIONS AND PHYSICAL. HISTORY 447 


Columbia. West of this range, the system includes much vol- 
canic rock, the greater part of which was extruded before the 
close of the period. The system is continued northward into 
Alaska,’ where it is less widespread than the Mississippian, so far 





Fig. 391. Section showing the position and relations of the Carboniferous sys- 
tem at a point in Colorado. AR=Archean; €=Cambrian; O=Ordovician; M= 
Mississippian; Cw and Cm=Carboniferous; J = Jurassic; Kd, Kb, Kn, and Km= 
Cretaceous. Length of section about 6 miles. (Eldridge, U. S. Geol. Surv.) 

















\\\ 
A 






Fig. 392. Section showing the Carboniferous in the Sierras of central Cali- 
fornia. C=Carboniferous; J (Mariposa slates)=Jurassic; mdi=metadiorite; 
ams = Amphibolite schist; V =igneous rock of various sorts, of Neocene age. Length 
of section about 6% miles. (Ransome, U. S. Geol. Surv.) 


as present knowledge goes. In the Arctic lands of America, the 
Mississippian and Pennsylvanian are not differentiated. One or 
both are widespread. | 

Thickness. The thickness of the system has a wide range, but 
like all preceding systems of the Paleozoic, it is thick (4,000 to 5,000 
feet) in the Appalachian Mountains. In the interior, it exceeds 
1,000 feet in but few places; but in Arkansas, the Coal Measures 
have been assigned the remarkable thickness of more than 18,000 
feet, from which it is inferred that there must have been land close 
at hand capable of supplying sediments in great quantity. This was 
probably the axis of the Ouachita uplift. In Texas, the thickness of 
the system ranges up to 5,000 feet, and in the west it is even thicker. 


Coal 
The general conditions under which sandstone, shale, and lime- 
stone originate have been outlined, but there has been no occasion 
heretofore to consider the formation of coal. From its economic 
importance, coal has been studied with more care than most sorts 
1 Brooks, Professional Paper 45. 


448 PENNSYLVANIAN PERIOD 


of rock, and geologists are agreed, in a general way at least, as to its 
mode of origin. . 

Origin. There is no doubt that coal is of vegetable origin. 
Except by the accumulation of vegetable matter, no way is known 
by which such beds of carbon could be brought into existence. 
Furthermore, the coal and its associated shales contain abundant 
remains of plants, in places even recognizable tree-trunks in the 
form of coal, and microscopic study has revealed the fact that much 
coal is but a mass of altered, though still recognizable vegetable 
tissues. Concerning the exact manner in which the beds of vege- 
table matter accumulated, and the conditions under which it was 
converted into coal, there is some difference of opinion. 

Much coal is essentially pure, containing little matter of any 
sort which was not in the plants which gave origin to it. Purity 
does not mean freedom from ash, since mineral matter, which on 
combustion becomes ash, is present in all plants. Along with the 
large amount of coal which is nearly pure, there is much which con- 
tains some earthy matter. Where the admixture of earthy matter 
is small, the coal is still usable; but from poor coal of this sort, there 
are all gradations into carbonaceous shale. 

The purity of some coal-beds over great areas warrants the 
conclusion that they were made of vegetation which grew where 
the coal is. The character of the vegetation shows that it grew 
on land or in swamps. Had it been washed down from its place of 
growth to the situations where the coal is, it should have been 
mixed with earthy sediment, and the product, after the necessary 
changes in the vegetable matter, would have been very unlike the 
purer coal-beds. Furthermore, the nearly uniform thickness of 
many of the coal-beds over great areas, some of them many thou- 
sand square miles, is a strong objection to the hypothesis that its 
substance was drifted together by any process whatsoever. 

Some other facts which support the theory that the vegetation 
grew where the coal-beds are, may be noted. (1) Beneath many 
coal-beds there is a layer of clay with roots (or root marks) in the 
position of growth. The clay seems to have been the soil in which 
the coal vegetation was rooted. (2) In association with the coal- 
beds, stumps of trees are found still standing as they grew (Fig. 
393). (3) In coal-beds, or in the associated layers of shale, imprints 
of the fronds of ferns or fern-like plants are found. They are in 
places so numerous and so perfect as to indicate that they were 


COAL 440) 





Fig. 393. Showing a stump standing as it grew in Coal Measures, near Glas- 
gow, Scotland. 


buried where they fell, without being drifted by moving waters from 
one place to another. (4) In many cases, the layer of rock next 
overlying a coal-bed contains abundant remains of vegetation, 
especially in its lower part, as if the conditions which brought about 
its deposition resulted in the destruction of the forest growth which 
had preceded. In such situations, trunks of trees 50 and 60 feet 
long, and 2 or 3 feet in diameter, have been found. 

While it is confidently believed that most of the workable coal 
represents the growth of vegetation im situ, it is not to be understood 
that coal was never formed from vegetation which drifted together. 

In the formation of a coal-bed, three things are to be accounted 
for: (1) The conditions under which the necessary quantities of 
vegetable matter accumulated; (2) how it was kept from decay; and 
(3) how changed into coal. 

Accumulation of organic matter. Large marshes, or marshes 
in low surroundings, are the only places where vegetable matter is 
now accumulating in quantity, with little admixture of sediment. 
Thus in the marshes along some parts of the Atlantic coast (Fig. 
394), there are quantities of organic matter which, locally, is mixed 
with little sediment. In Dismal Swamp, the stems, branches, 
leaves, and fruits of the trees, shrubs, and herbs which grow there, 


450 PENNSYLVANIAN PERIOD 


have been long accumulating, and little sediment is mixed with 
them. In cypress and mangrove swamps, too, there are consider- 
able thicknesses of vegetable matter nearly free from mud, etc. 
The multitude of marshes and peat- aean in the United States and 
Canada are fur- 
ther illustrations 
of the accumula- 
tion of vegetable 
matter, in some 
cases mixed with 
abundant sedi- 
| ment and in 
some nearly free 
from it. 

The vegetation 
| in swamps need 
not be more 
luxuriant than 
| that on moist 
| lands which are 
} not swampy. On 
| fertile prairies 
— ————4 and in some for- 


ae a 394. Map of the Cape May peninsula, showing ests the annual 
coastal marshes. The unshaded areas inside the coast line growth of vege- 
are dry land. 

















tation is great; 
but since the leaves, fruits, twigs, and trunks decay as they fall, 
the larger part of their substance is returned to the atmosphere. 
In a moist region there is more growth (and therefore more death) 
of vegetation than in a dry one, and a better chance that decay will 
not keep pace with death. 

Preservation of vegetable matter. Where vegetation falls 
into water, as in marshes, it undergoes slow change different from 
the decay suffered by vegetation on dry land. It is the partial 
preservation of organic matter in the water of marshes and ponds 
which converts them into peat-bogs, for peat is nothing more than 
accumulated vegetable matter undergoing those changes to which 
vegetable matter in water is subject. Under favorable conditions, 
the peat of a bog may become very deep, as in the Dismal Swamp. 
In and about marshes and swamps, therefore, we find the conditions 


COAL 451 


for the accumulation of considerable thicknesses of vegetable mat- 
ter, some of it nearly free from sediment, and at the same time the 
conditions which keep it from complete decay. 

Conversion into coal. While the vegetable matter is not de- 
stroyed, it is not preserved intact. The approximate composition 
of wood and peat are shown by the following analyses (ash omitted). 


Carbon Hydrogen Oxygen Nitrogen 
SLi ye ae 49 .66 6.21 43.03 I.I0 
MEAD ete ct 59.50 Sete 33.00 2000 


The relative atomic proportions of carbon, hydrogen, and oxy- 
gen in cellulose are expressed by the formula (CsHO;),. In the 
air, the carbon and the hydrogen of the wood unite with oxygen 
of the air or of the wood itself, forming carbon dioxide and water, 
the principal products of the decay of vegetation. But under water 
the atmospheric oxygen is largely excluded, and the elements of 
the wood are thought to unite with one another to a larger extent, 
while the oxygen of the air plays but a subordinate part. One of 
the common products of decay under such circumstances is CH, 
(marsh-gas), which escapes into the air. The formation of this gas 
exhausts the hydrogen of the organic matter four times as rapidly 
as the carbon. If the carbon and oxygen of the wood are given off 
combined as COs, the oxygen is consumed twice as fast as the car- 
bon. If the hydrogen and oxygen of the wood are liberated as 
water, the result is to increase the proportion of carbon remaining. 

While the exact quantitative relations of the reactions which 
take place are not known, and are probably not constant, the fol- 
lowing table ' suggests certain changes which might take place, and 
the products which would remain at certain stages: 


12 CgHi0Os (cellulose) — 8 CO. = Cg2H72004 (peat). 
Ce2H72004 (peat)— = C57H56010 (brown coa!), 
C57H56010 (brown coal)— 


= C54H 420; (bituminous coal). 


= C4gHjgO0 (anthracite coal). 


Ww 
a 
jo) 
ee ee et ee ee 


C54H4205 (bituminous coal)— CO, 
5 CHg 


1 Prepared by Rollin T. Chamberlin. 


462 PENNSYLVANIAN PERIOD 


From this table it will be seen that the process which converts 
vegetable matter into coal is characterized by progressive changes 
in the nature of the chemical decomposition. The elimination of 
hydrogen and oxygen (H2O) probably is the dominant change in the 
production of peat from cellulose. Second in importance at this 
stage is the removal of oxygen in the form of COs, while the libera- 
tion of methane (CH) is of still less importance. As the alteration 
of the peaty material progresses through successive stages to coal, 
less and less water and carbon dioxide are given off, and there is an 
increase in the proportion of CH, set free. Laboratory investiga- 
tions have shown that while CO, may constitute an important part 
of the free gas held in the pores of some of the Cretaceous coals, the 
gas which escapes from the more advanced stages of Pennsylvanian 
anthracite coal is largely CH4. The burial of the peat compresses 
it, and the physical change resulting is a part of the process of coal- 
making. 

If coal-beds represent former swamps, as they are believed to, 
we have still to inquire into the conditions under which such ex- 
tensive swamps existed, and to seek the explanation of their recur- 
rence (one for each coal-bed) in many regions. 

The first condition for a swamp is lack of drainage, and the 
second a sufficient, but not an excessive amount of water. Enough 
to stop the growth of vegetation would be excessive, and too little 
to preserve it after its growth and death, would be insufficient. 

During the widespread movements which affected the eastern 
interior at the close of the Mississippian period, great areas appear 
to have emerged from the sea. Early in the Pennsylvanian period, 
considerable tracts which were not submerged stood so low as to 
be ill-drained, or undrained, and constituted marshes. Climatic 
conditions were such as to permit the growth of abundant vegetation 
in the marshes, where, after death, the vegetable matter underwent 
changes of the nature suggested above. The marshes were thus 
converted into peat bogs. Some of the great coal-swamps prob- 
ably came into existence along shores, and some in shallow inland 
basins or undrained areas. 

Each coal-bed represents the accumulated vegetable growth 
of a long period. It would appear that the growth and accumula- 
tion of vegetation was repeatedly brought to an end by subsidence 
which let the water (sea, lake, or aggrading stream) in over the 
marshes, drowning the plants, and burying the organic matter which 


COAL 453 


had already accumulated under sediment which the submergence 
brought in its train.. A second coal-bed in the same region points 
to the recurrence of swamp conditions, and means either (a) that 
after submergence and burial of the organic matter, slight emergence 
reproduced the conditions for bogs; or (0) that by sedimentation the 
sea or lake bottom where the first bog had been was built up to 
water-level, restoring swamp conditions. 

The number of coal-beds is, in many places, great. In some 
parts of Pennsylvania it exceeds 20; in Alabama, 35 (not all 
workable); in Nova Scotia (including some dirt-beds) about 80; 
but in the Mississippi basin west of the Appalachians, the number 
is in most places less than a dozen. In Illinois the workable beds 
are nine. 

Extent and relations of coal-beds. The widespread distribution 
of coal does not mean that any one marsh necessarily covered the 
whole of any one great coal-field. Some coal-beds, however, are of 
great extent. Thus the Pittsburgh bed is worked over an area of 
some 6,000 square miles! in western Pennsylvania, Ohio, and West 
Virginia, and has at least an equal extent where too poor to be gen- 
erally productive. Many coal-beds, on the other hand, are not 
extensive. From their thicker portions they thin out in all direc- 
tions, grading into black shale in many places. Many facts sug- 
gest that within the general area of a coal-swamp there may have 
been elevations (islands), interrupting the continuity of the swamps, 
and therefore of the coal-beds. 

Varieties of coal. The ways in which the different varieties 
of coal arose have never been determined precisely. In general, 
anthracite coal occurs in mountainous regions, where the coal and 
other layers of rock with which it is associated have been subject to 
much dynamic action. Thus, in the mountains of eastern Penn- 
sylvania (Fig. 384) the coal is mainly anthracite, while in’ other 
coal-fields of the same age, where the strata are deformed much less, 
the coal is bituminous. In Arkansas, where the strata have been 
subject to some, but not to extreme dynamic action, the coal is semi- 
anthracitic.2, Where the dynamic metamorphism of the associated 
rock has been great, as in Rhode Island, the coal has gone beyond 
the anthracitic stage. Anthracite coal is found also in some places 
(not in the Coal Measures of the United States) in contact with 


1 White, West Virginia Geol. Surv., Vol. II, p. 166. 
2 Ann. Rept. Ark. Geol. Surv., 1888, Vol. III. 


454 PENNSYLVANIAN PERIOD 


intrusions of igneous rock. Other sorts of sedimentary rock axe 
metamorphosed in similar situations. 

These phenomena suggest that anthracite is metamorphic coal, 
produced from bituminous coal by processes similar to some of those 
which metamorphose other sorts of rock. The fact that most 
metamorphic coal is found in regions where erosion has exposed its 
beds (Fig. 385) led to the conjecture that exposure of the coal might. 
be a factor in the problem, the exposure favoring the escape of the 
volatile constituents, and so aiding in the transformation of soft 
coalinto hard. Some beds of bituminous coal are, however, exposed 
freely. Both dynamic action, involving pressure and heat, and 
exposure would seem to be conditions favoring the development of 
anthracite, but it does not follow that these are the only factors in 
the problem, or that anthracite coal has never been produced in 
other ways. White has advanced the idea that deep-seated, hori- 
zontal thrust movements are the essential cause of devolatilization!. 

There are several varieties of bituminous (soft) coal, some of 
which appear to depend on the nature and extent of the decay of 
the vegetable matter before its burial, and some on the degree to 
which the devolatilizing processes have been carried since burial. 
Recent studies seem to indicate that the kind of vegetation enter- 
ing into the coal may have an important effect on the product. 
Some coal seems to be made up largely of alge, or of the spore- 
cases of certain plants, and such coal has rather distinctive quali- 
ties, if recent interpretations are correct. 


Other Products of Economic Value 


The iron ore of the Coal Measures occurs in layers, or in the 
form of nodules concentrated at a given horizon, forming a nearly 
continuous layer. The iron of the Coal Measures seems to have 
been deposited largely as a precipitate from the waters of inland and 
local basins while the other members of the system were being laid 
down. Dissolved by the land waters from the soil and rocks, it 
was brought to the marshes in some soluble form. In the marshes, 
it was precipitated in the form of iron carbonate or iron oxide. Sub- 
sequent oxidation has changed some of the original carbonate into 
the oxide. The principal iron ores of the system occur in Penn- 
sylvania and eastern Ohio. The system yields oi] and gas in some 
places, as in Oklahoma, Kansas, and Illinois. 

1 David White. Economic Geology, Vol. III. 


GENERAL CONSIDERATIONS 455 


General Considerations 


Geographic conditions in the eastern interior. Returning to 
the system of which the coal-beds form a small part, it is to be 
recalled that the formations represent an alternation of marine, 
lacustrine, and marsh conditions. The cause of the alternation was 
probably geographic, but it is not to be inferred that geographic 
changes were more frequent at this time than during other periods. 
Their record is conspicuous because the land was near sea-level, so 
that extensive submergence and emergence resulted from slight 
changes of relative level of land and sea. Equally frequent and 
equally extensive movements would leave no such record of them- 
selves, if the surfaces concerned were far above or far below sea- 
level. It was oscillation just above and just below water-level (or 
base-level) which allowed the record to be so clearly preserved. 
How far the oscillations were due to warpings of the land, and how 
far to changes in the level of the sea, cannot be determined; but when 
we recall that the ocean-level must respond to every deformation 
which affects its bottom, and to every stage of filling, it is strange 
that its level is in a nearly perpetual state of change. 

In general, it may be said that the movements of the crust 
which have been of most importance, from the point of view of 
continental or biological evolution, are not those which have 
affected high land or deep sea bottom, but those which have con- 
verted sea bottom into land, or land into sea bottom. Such changes 
are most likely to have taken place where land was low, or water 
shallow. From the point of view of geology, therefore, the critical 
level of crustal oscillation is the level of the sea. 

Duration of the period. So uncertain is our knowledge of the 
duration of geological time that all sorts of data which can be made 
to throw light on the subject are of interest, even though they do 
not lead to trustworthy numerical conclusions. Under favorable 
conditions, a foot of peat may accumulate in ten years or even less; 
but the common rate is probably much slower. A vigorous growth of 
vegetation has been estimated to yield annually about one ton of 
dried vegetable matter per acre, or 640 tons per square mile. If 
this annual growth of vegetable matter were all preserved for 1,000 
years, and compressed until its specific gravity was 1.4 (about the 
average for coal) it would form a layer about seven inches thick. 
But it has been estimated that four-fifths of the vegetable matter in 


456 PENNSYLVANIAN PERIOD 


peat bogs escapes as gas (COs, CHu, etc.), while the peat is being 
changed to coal. If this is true, the seven-inch layer would be re- 
duced to less than one and one-half inches, and a layer one foot in 
thickness would require between 8,000 and g,oo00 years.. The aggre- 
gate thickness of coal is as much as Ioo feet in many places, and as 
much as 250 feet in some. At the above rate of accumulation, 
periods ranging from nearly 1,000,000 to nearly 2,500,000 years 
would be needed for such thicknesses. It should be borne in mind, 
however, that much depends on the rate of growth of Carboniferous 
vegetation, which is not known. 

On the other hand, these figures refer to the coal only, not to 
the Coal Measures. The greater part of the Coal Measures is shale 
and sandstone, and of these formations there are thousands of feet, 
even where the sediments were fine and their accumulation prob- 
ably slow. It would hardly seem unreasonable to conjecture that 
their deposition may have consumed as much time as that of the 
coal. Doubling the above figures, we get 2,000,000 and 5,000,000 
years respectively, figures which must be taken to mean nothing 
more than that the best data now at hand indicate that the Penn- 
sylvanian period was very long. 

Close of the period. After the long period of oscillation above 
and below the critical level recorded by the Coal Measures, the 
interior east of the Mississippi was brought above the level of the 
sea, not to sink beneath it again during the Paleozoic era, and some 
of it at no later time. This emergence marks the close of the 
Carboniferous, and the inauguration of the Permian period. It is 
also probable that the deformative movements which were to 
develop the Appalachian Mountains began at this time. There 
were notable changes also in the western half of the continent, for 
the Permian system is much less widespread than the Pennsyl- 
vanian. Where the Permian occurs, its constitution and fossils 
indicate not only different relations of land and water, but different 
conditions of erosion. 


Foreign Countries 


Europe. As in America, the oldest formation of the Upper 
Carboniferous in Europe is in many places a conglomerate and sand- 
stone formation, called the Millstone grit, in England. The Coal 
Measures consist principally of shales, with sandstone and limestone. 
Associated with these commoner sorts of rock, there are beds of coal 


IN FOREIGN COUNTRIES 457 


and clay-iron-stone, both of which occupy positions corresponding, 
in essential respects, with those of similar formations in eastern 
North America. There is workable coal in Great Britain, Ireland, 
Belgium, France, Spain, Germany, Austria, and Russia, but the 
total area of productive coal in Europe is much less than in America. 

In Russia, as already noted, the Lower Carboniferous (Missis- 
sippian) contains much coal, while the Upper is chiefly of limestone; 
but in southern Russia (Donetz coal-field) there is coal in the Upper 
(Pennsylvanian) division. The Upper Carboniferous limestone of 
Russia (Fusulina limestone) is similar to that of southern Europe. 
The faunas of the marine part of the system in Europe have much 
likeness to those of western North America, suggesting that marine 
life was able to pass between these continents, via northern Asia. 

Igneous rocks are associated with the Upper Carboniferous 
formations of sedimentary origin in western Europe. Their extru- 
sion seems to have been an accompaniment of the crustal disturb- 
ances which affected western Europe in the course of this period. 
These movements appear to have been greatest during the Upper 
Carboniferous period (after the Westphalian epoch). 

Other continents. The Upper Carboniferous of Asza is repre- 
sented by both marine and non-marine formations. The non- 
marine phase, with numerous beds of coal, is found in Asia Minor, 
on the east side of the Middle Urals, and in northern and eastern 
China, reaching to northern Tibet on the one side, and to Mongolia 
on the other. The Carboniferous of some parts of China contains 
coal-beds of great thickness. The system is also present in India. 

The Carboniferous formations of northern Africa are similar to 
those of southern Europe, but in southeastern Africa, a coal basin 
has been reported in Zambesi.! | 

The Carboniferous system is well developed in Australia where 
it is not in all places clearly separated from the Permian. Both the 
Carboniferous and the Permo-Carboniferous systems contain coal. 

In South America, rocks of Late Carboniferous age are somewhat 
widely distributed. In southern Brazil they contain much coal.’ 
The system is widespread in the lower part of the basin of the 
Amazon, where it rests on older formations unconformably, and is 
not generally coal-bearing. 


1 Kayser, Geologische Formationskunde, p. 207. 
2 White, I. C, Commissdo de Estudos das Minas de Carvaéo de Pedra do Brazil, 
1908, 


458 PENNSYLVANIAN PERIOD 


LIFE 


With this period the chief biological interest shifts from sea to 
land, and centers in the vegetation and the amphibians. 


Plants 


Plant life was very abundant in this period, and its record is 
unusually full and perfect. Its completeness has doubtless given 
this flora an undue prominence over those which preceded and suc- 
ceeded it; yet it was really a great period in the history of plant life. 
Angiosperms (flowering plants, p. 685), the dominant plants to-day, 
had not yet appeared, but gymnosperms (the group to which pines 
belong) were abundant, and pteridophytes (ferns and related plants) 
probably made their greatest display at this time. All the great 
divisions of this group (p. 685) were present, and all of them were 
nearly or quite at their climax. Of lower plants, little is known. 
The most rapid evolution of floras was perhaps in the Pottsville 
epoch. Half the genera of that epoch scarcely survived it, and few 
of them lived after the Allegheny epoch. 

The early floras were widely distributed. Thus three floras in 
Asia Minor may be correlated severally with three floras of the 
Pottsville series. The place of origin of these early floras is not 
known with certainty, but present evidence points to western 
Europe and eastern North America, with an Arctic land connec- 
tion. The late Pennsylvania floras are less sharply separated from 
the early ones in North America than in Europe. The later floras 
indicate greater diversity of climate than the earlier. 

The dominant plants of the period belonged to five groups: 
(1) the horse-tail family (Equisete), (2) sphenophylls, now extinct, 
(3) lycopods, or club mosses, (4) fern-like plants (pteridosperms), 
and (5) Cordaites, a group of gymnosperms. To this list ferns 
should perhaps be added. 

Ferns were a minor element of the Pennsylvanian flora, though 
fern-like leaves are the most abundant of the plant fossils. It is 
now known that most of them belonged to seed-bearing plants and 
not to ferns. Nevertheless, true ferns were present. Species still 
live which, so far as outward form is concerned, might be referred 
to Carboniferous genera. 

The horse-tail group ( Equisetales) was represented by calamites 
(tree horse-tails), a conspicuous element in the Pennsylvanian flora. 
They must have been graceful trees, of the same general habit as 


LIFE 459 


modern horse-tails, except for their large size. The largest modern 
tropical representatives of the group have slender stems 30 or 4o 
feet high, whereas the Pennsylvanian calamites reached a foot or 
two in diameter and probably 60 to go feet in height. They had 
hollow stems, or a core of pith, and casts of the interior are common. 
Branches from the trunk were comparatively few, and in whorls. 





Fig. 395. A composite group of leading CARBONIFEROUS PLANTs, adapted 
from restorations by various paleobotanists, by Mildred Marvin. In the fore- 
ground at the right, Lepidodendron; at the left Sigillaria; in the right center rear, a 
tree fern; in the left center rear, Cordaites; at the extreme right and left, Calamites. 


The leaves also were in whorls (Fig. 395) and dwarfed, though 
larger than in the modern type. Their roots were of the type 
commonly found under water or in wet places, and the calamites 
probably frequented swamps and lowlands. Roots are known to 
have been sent out as much as nine feet above the base of the stem. 


460 PENNSYLVANIAN PERIOD 


They were probably associated in thickets and jungles, like cane- 
brakes and bamboos. Their history may run far back, as they were 
well differentiated in the Devonian; but their ancestry is uncertain. 
The stems of adult calamites are so unlike those of modern horse- 
tails that their kinship was long unrecognized, and calamites were 
thought to be gymnosperms; but it is now known that the stems of 





d 


Fig. 396. Group oF FERN Fronps: 4a, Neuropieris auriculata, Brgt.; b, N. 
angustifolia, Brgt.; c, N. vermicularis, Lx.; d, Odontopteris cornuta, Lx.; e, Pecopteris 
unita, Brgt.; f, Dictyopteris rubelia, Lx.; g, Archeopteris bochsiana, Goepp.; h, 
Sphenopteris splendens, Lx. 


the young plants had the same general structure as the horse-tails, 
and that the gymnospermous features belonged to the later stages 
of the life of the individual plants. The group is represented to-day 
by one genus (Eguisetum) and about 20 species. Its evolution 


LIFE 401 


records a continuous decline from the Pennsylvanian period, when 
it was at its best. 

Recent studies have shown that the graceful, slender plants with 
whorled leaves, referred to the genus Sphenophyllum (c, Fig. 397), 
formerly classed as calam- am x | 
ites, should be made a class 
(Sphenophyllales, p. 685) by 
themselves. Their interest 
lies chiefly in the fact that 
while they have certain cal- 
amarian features, they have 
others possessed by lyco- 
pods. This is interpreted 
to mean that these two 
yroups (calamarians and ly- 
copods) were united with 
Sphenophyllales in a com- 
mon ancestral form.! The 
stems were long, slender, and 
apparently weak, and a 
climbing habit has been in- 
ferred. The leaf structure 
suggests a shady habitat, 
perhaps one of undergrowth. 
The class, represented in 
the Devonian, had its climax 
in the middle Pennsylvan- a 
ian, and continued into the Kj 

: ‘ ig. 397. CARBONIFEROUS EQUISETALES 
Permian and possibly later. np SpHenopnyiiates: a, Calamites cistii; 

In size Lycopods (p. 685) 5, Annularia sphenophylloides; c, Spheno- 
were the master group of phyllum longifolium. 
the Coal flora. ‘They were represented by trees of large size 
which had the highest organization reached by the pteridophytes. 
From this high estate, they have since fallen to prostrate 
or weakly ascending plants of moss-like aspect (club mosses 
and ground pines.) The chief genera were Lepidodendron and 
Sigillaria (Fig. 395), of which the former was the earlier and sim- 
pler type. Both take their names from the leaf-scars (lepidos = 
scale, sigilla=seal) which the trunks retained (Figs. 398 and 399). 

1 Seward, Fossil Plants, p. 413; Scott, Studies in Fossil Botany, p. 494. 





462 PENNSYLVANIAN PERIOD 


The trunks of some lepidodendrons were too feet in length. 
They were erect, and branched at a great height. The leaves were 
linear or needle-shaped, ranging up to six or seven inches in length, 
and set densely on the branches. Some of them had characteristics 





Fig. 398. Leaf markings Fig. 399. Leaf markings of a 
of a lepidodendron. sigillarian. 


pointing in the direction of seeds, but it is not known that seed- 
producing plants sprang from them. More than too species of 
lepidodendrons have been described. They seem to have reached 
their climax early in the period, and nearly all had disappeared by 
its close. . 

The sigillarians differed from the lepidodendrons in being with- 
out many branches. They were perhaps the largest of the trees, 
their trunks reaching six feet in diameter, and 100 feet or more in 
height. The stems were densely clothed with erect, rigid, linear 
leaves. They were more abundant than lepidodendrons before the 
close of the period, but were on the wane at its close. 

The group is essentially Pennsylvanian but initial forms lived 
in the Devonian and Lower Carboniferous, and a few survived to 
the Permian. 

Cordaitales. One of the characteristic trees of the period was 
Cordaites, which belonged to a remarkable family (now extinct) of 
gymnosperms. ‘The trees were go feet or more in height, and rather 
slender. The wood was of the coniferous type, covered, as in so 
many other plants of the period, by a thick bark. ‘The trunks had 
a large pith. The leaves were parallel veined, suggestive of mono- 
cotyls of the yucca type, and in some cases attained a length of six 


—_" 


LIFE 463 





Fig. 400. One of the Cycadofilices, Lyginodendron oldhamia. (Restoration 
by D. H. Scott and J. Allen.) 


feet and a width of six inches. They are preserved in great abun- 
dance, and make up a large part of some beds of coal. In one form, 
the leaf had a distinctly fleshy character, as if adapted to xerophytic 
(dry) life. The florai organs were peculiar to the family, and have 
been worked out with marvelous success, even the structure of the 
pollen having been determined. 

Conifers have not been found in the Pennsylvanian rocks; but 
the vegetation of the uplands, where conifers probably would have 
lived, is not known. 


Climatic Implications of the Coal-plants 


What suggestions do the Coal-plants give relative to the atmos- 
pheric conditions under which they grew? ‘Two partly antagonistic 
views relative to these conditions have been held. ‘The one regards 
the beds of coal as evidence of a very luxuriant growth of vegeta- 
tion, which in turn has been thought to imply a warm, moist atmos- 
phere, heavily charged with carbon dioxide. The great size of 


464 PENNSYLVANIAN PERIOD 


many of the trees, the succulent nature of many of the plants, and 
the abundance of aérial roots, are appealed to as evidence of mild- 
ness of climate, while the absence of rings in the wood, and, above 
all, the distribution of similar floras through diverse latitudes, point 
strongly to an equable climate, especially in the earlier part of the 
period. It is clear that this view has much support. 

The alternative view postulates less warmth and moisture, and 
more diversity; in other words, a nearer approach to the present 
conditions. It assumes, however, a somewhat higher percentage of 
carbon dioxide than now, and a climate milder and more uniform 
than that of to-day. The basis of this view is found in the following 
considerations: (1) Great thicknesses of coal do not necessarily 
imply rapid accumulation, any more than great thicknesses of lime- 
stone do. Given favorable conditions of preservation, slow growth 
will produce great thicknesses. (2) At present the accumulation of 
peat, the nearest analogue of coal formation, is most favored in 
cool climates, and is taking place chiefly in high latitudes. (3) The 
dominant plants had narrow leaves with their breathing pores con- 
fined to deep furrows on the under side, devices common to plants of 
dry regions. (4) The trees had thick corky bark, as though protec- 
tion from external conditions was needed. 

The thickness of the bark, and the form and structure of the 
leaves, give a xerophytic aspect to the overgrowth made up of 
lepidodendrons, sigillarias, calamites, and cordaites. This is not 
the case with the undergrowth, but this would not be expected of 
shaded plants. The force of the inference from the xerophytic 
aspect of the overgrowth is much weakened by the fact that the 
vegetation of undrained swamps and bogs has many xerophytic 
features. It is clear that a more critical study of the problem is 
needed before a final conclusion concerning the climate of the period 
is reached. 

Land Animals 

So far as the evolution of air-breathing vertebrates is concerned, 
this is one of the most important periods in geological history.! 
Amphibians, insects, spiders, scorpions, and myriapods, lived on the 
land at this time. The amphibians are perhaps of chief interest, 
for they were the first land vertebrates. 

The rise of amphibians. Tracks attributed to amphibians are 
found in the Devonian and Mississippian, but in neither of these 

1 Williston. Faunal Relations of Early Vertebrates, Jour. Geol., Vol. xvii, p. 389. 


LIFE 465 


systems have bones of these animals been found in America, and 
only imperfect ones in Europe. Fossils of amphibians first appear 
in abundance in the later Coal Measures, and in such variety as 
to imply a long antecedent existence. Most of them were rather 
primitive in structure, but they were ee 

genuine amphibians, not transition a 

types. All of them seem to have 
had elongate forms, and their heads 
were well roofed over by the bony 
plates of the skull. On account of 
this last feature they are called 
stegocephalians (roofheaded). Some 
of them have also been named /aby- 
rinthodonts, from the intricate infold- : ) a 
ing of the dentine of their teeth. Hee ew aM wl a 
Labyrinthodonts were doubtless the i Se on . y y 
largest amphibia of the period, some Ds 

of their skulls reaching a length of 
half a meter. 

The amphibia varied in length 
and strength of limb, in agility, 
ability to climb, etc. The elonga- 
tion of their bodies involved a nota- 
ble multiplication of the vertebre, 
one form having no less than 150. 
Before the close of the period, prob- 
ably some of them lived on dry land 
where fleetness, rather than protect- 
ive armor, preserved them from their 
enemies. Others were limbless and 


SS 

















=a 





































































































































































































































































































i} pit i 
i inh 





snake-like, crawling reptiles in every- 

: ; ; : : Fig. 401. A CARBONIFEROUS AM- 
thing except certain technical details PPE MT bor. dobbsig uxt 
of their palates. ley. A microsaurian from Kil- 


One branch of the amphibia_ kenny, Ireland, about 3/5 natural 
which reached its highest develop- ‘i: (4ittel.) 
ment in the Permian is supposed by some paleontologists to be the 
ancestral stock from which mammals arose. The other branch, 
which included the labyrinthodonts, is the only group of Pennsyl- 
vanian air-breathing vertebrates which left no descendants. 

Not much is known of the habits of the amphibia, but from their 


466 PENNSYLVANIAN PERIOD 


teeth it is inferred that they were predaceous. In Nova Scotia, 
Dawson took thirteen skeletons of amphibians from a single sigil- 
larian stump. Since land shells and myriapods are found in stumps 
with the amphibian skeletons, it has been inferred that some of the 
amphibians were climbers, and lived on mollusks, myriapods, and 
similar land life. 

The amphibians of different continents were so similar as to 
suggest great freedom of communication and migration, but free 


1 





Fig. 402. CARBONIFEROUS TERRESTRIAL AND FRESH-WATER LIFE. Plants: 
a, Callipteridium mansfieldi Lesq., b and c, Callipteridium membranaceum Lesq., 
species of ferns. Land shells: d, Zonites priscus Carp., e, Pupa vermilionensis 
Bradley. These land snails have been referred to genera living at the present time, 
and although this reference may eventually prove to be incorrect, they are at least 
close relatives of recent genera. Insects, etc.: f, Euphoberia armigera M. and W., 
a Carboniferous myriapod or thousand-legged worm; g, Eoscorpius carbonarius 
M. and W., a scorpion very similar in type to living forms; 4, Arthrolycosa antiqua 
Harger, a spider more primitive than recent forms as seen by the segmentation of 
the abdomen; 7, Progonoblattina columbiana Scudd., one of the allies of the modern 
cockroaches which were the most conspicuous members of the Carboniferous insect 
fauna. Crustacea: j, Anthrapalemon gracilis M. and W., k, Pale@ocaris typus 
M. and W., types of crustaceans found in the Mazon Creek nodules; /, Prestwichia 
dane M. and W., an early ally of the modern horseshoe crab. (Weller.) 


LIFE 467 


intercontinental migration seems to have come to an end by the 
close of the period. 

Insects. Hundreds of species of insects have been identified 
from the Coal Measures. They were, for the most part, rather 
primitive types. Orthopters (cockroaches, 7, Fig. 402, locusts, 
crickets, etc.) were greatly in the lead, followed by neuro pters (repre- 
sented by ancestral mayflies). These two orders include about 90 
per cent of the known insects. Hemipters (bugs), which had ap- 
peared earlier, and possibly coleopiers (beetles) were present, but no 
fossils of bees, butterflies, or moths have been found, and there is 
little probability that they existed, since flowering plants, on which 
they depend, had not yet appeared. There is no record of flies. 
The evolution of insects was therefore one-sided. Curious forms 
were developed within the orders which lived, and remarkable sizes 
were attained, spreads of wing of a foot or more being reported. 

Spiders and myriapods (Fig. 402) were plentiful, and several 
species of Jand snails (d and e) have 
been identified. The amount of car- 
bon dioxide in the atmosphere could 
not have exceeded that compatible 
with this varied assemblage of air- 
breathing life. : 


- Fresh-water Life 


Besides fresh-water plants, the 
life of land waters appears to have 
consisted of fishes, mollusks, crus- 
taceans, and doubtless of many other 
forms. Aside from the development 
of the fresh-water fish and amphibia, 
perhaps the most suggestive feature 
was the association of the arthropods 
with other forms of life. Eurypterids 
(Fig. 403) were still in existence, and 
their relics are so intimately associ- 





; y ; Fig. 403. Natural association 
ated with ferns, calamites, insects, of Eurypterus mansfieldi with 


spiders, and scorpions as to leave no _ ferns and calamites. (From Dana 


reasonable doubt that they were atter Hall.) 


fresh-water forms. ‘There were also crustaceans resembling cray- 
fish, and others of shrimp-like appearance. 


468 PENNSYLVANIAN PERIOD 


Marine Life 


Two phases of sea life are worthy of note, (1) that which occupied 
the shallow water, which, in the form of estuaries, lagoons, and 
shoals, crept in and out on the borders of the continent as the rela- 
tions of land and sea oscillated, and (2) the life of the more open seas. 
No doubt this distinction had existed always, but it had not before 
reached equal importance. In the coal regions of this period, a large 
part of the fossils are of shallow water types. In shallow water, 
where sandy and muddy flats prevailed, pelecypods and gastropods, 
together with certain fishes, predominated, while in the more open 
seas the brachiopods, cephalopods, and clear-water types were 
more plentiful. During the period, there was progress among the 
fishes in adaptation to swift movement, and in shapeliness of form. 

It is difficult to tell which of them were marine, which fresh water, 
and which common to salt and fresh water. It is clear that much the 
larger number of those in the American Coal Measures lived in 
fresh water; whether also in salt water is uncertain. 

Fig. 404 shows a group of Pennsylvanian marine fossils. It 
may be noted that ancient and relatively modern types of cephalo- 
pods lived together, the former represented by straight, plain, small 
orthoceratites (z, Fig. 404), and the latter by closely coiled goniatites 
(sz), with curved sutures. The former were about to take their 
final leave, and the goniatites were about to evolve into ammonites, 
the dominant type of the Mesozoic era. Brachiopods were abun- 
dant, and their general facies was like that of the later Mississippian. 
Some species range not only through northern America and Eurasia, 
but into the Oriént and Australasia. A close relation between sev- 
eral American and Russian crinoids implies intermigration. Cyst- 
oids and blastoids were gone, and other forms of echinoderms were 
rare. Tvrilobites, which commanded foremost attention at the 
opening of the Paleozoic, are now almost at the point of disappear- 
ance. ‘The last representative of the group had the chaste beauty 
of its early ancestors. Bryozoans were not uncommon, but the 
peculiar devices for support illustrated in Archimedes and Lyropora 
of the preceding period were abandoned. Protozoans were repre- 
sented widely by a little foraminiferal shell (Fusulina secalicus, 6, 
Fig. 404), which had about the size and form of a grain of wheat. 
Its abundance gives character to the Fusulina limestone which occurs 
in America, Europe, and Asia. Corals were rare, as might be ex- 
pected under the conditions of the time. 


- 
= 
> 
- 
- 
Sa 
~ 
- 
” 


NETL LL PIETER iELE iAH 





Fig. 404. PENNSYLVANIAN MARINE Fauna: a, Eupachycrinus magister M. 
and G., a crinoid with biserial arms; 6, Fusulina secalicus Say, a foraminifer shell 
that in places makes up considerable beds of limestone. c-p, brachiopods: , 
Productus nebrascensis Owen; d, Productus costatus Sow.; e, Seminula argentea 
(Shep.), a spire-bearing common Carboniferous species; f, Lingula umbonata Cox, a 
representative of a genus which persisted from the Cambrian to recent times; g, 
Hustedia mormoni (Marc.); h, Spirifer cameratus Mort., a characteristic member of 
the Pennsylvanian fauna; i, Productus symmetricus McCh.; j, Derbyia crassa (M. 
and H.); k, Enteletes hemiplicata (Hall); 1, Pugnax uta (Marc.); m, Dielasma bovidens 
(Mort.); 2, Meekella striatocostata (Cox); 0, Chonetes granulifera Owen; p, S piriferina 
kentuckiensis (Shum.). 7, Allorisma subcuneata M. and H. 4q-t, pelecypods: gq, 
Monopteria longispina Cox; s, Myalina recurvirostris M. and W.,; t, Aviculopecten 
occidentalis Shum. u-x, gastropods: u, Worthenia tabulata (Con.); v, Meekospira 
peracuta (M. and W.); w, Bellerophon percarinatus Con.; x, Naticopsis altonensis 
(McCh.); y, z, and zz, cephalopods: y, Temnocheilus forbesianus (McCh.); 2, Ortho- 
ceras cribrosum Gein.; 22, Paralegoceras newsomi Smith; «x, Phillipsia major Shum. 


Map work. No reference to map work has been made since that at the close 
of the chapter on the Ordovician, p. 387. Experience has shown that if the prin- 
ciples of stratigraphy, as illustrated by the Cambrian system, are well developed, 
further map work may be deferred to about this point. See laboratory manual al- 
ready referred to (p. 387), exercise IX. 


469 


CHAPTER XXI 
THE PERMIAN PERIOD 


FORMATIONS AND PHYSICAL HISTORY 


At the close of the Pennsylvanian period much of the central 
and eastern parts of the United States became dry land, and the 
sea-covered area in the west was greatly restricted. The area of 
land was perhaps as large as at any time since the beginning of the 
Paleozoic. The waters which still lay upon the continent were 
partly in the form of lakes and inland seas, and partly connected 
with the open ocean; but the areas which the sea overspread at the 
beginning of the period were largely abandoned before its close. 
These changes in geography reflected themselves both in the distri- 
bution of the Permian formations and in their character. 

East of the Mississippi. During the earlier part of the period 
fresh-water sedimentation continued much as before in some parts 
of the east (parts of Pennsylvania, West Virginia, Maryland, and 
Ohio), and with the commoner sorts of sedimentary rocks there is 
some coal. There, and in and about Nova Scotia, non-marine 
Permian strata rest on Pennsylvanian beds in such a way as to show 
that sedimentation was not seriously interrupted. The systems are 
separated on the basis of fossils. Recently, a conglomerate forma- 
tion (the Roxbury) of Eastern Massachusetts has been interpreted as 
of glacial origin. This origin suggests its reference to the Permian!. 

West of the Mississippi. West of the Mississippi the system is 
better developed, being partly marine and partly non-marine. In 
Kansas and Nebraska its lower part is marine, and the Permian ~ 
of these states is probably continued northwestward to. Wyoming 
and South Dakota. The marine Permian of Kansas is overlain by 
beds containing gypsum and salt, and possessing other features 
which show that the open sea of the region was succeeded by dis- 
severed remnants, or by salt lakes whose supply of fresh water was 
exceeded by evaporation. With the saline and gypsiferous deposits, 
and above them, are the ‘‘ Red Beds,”’ many of which are Permian. 


1 Sayles, Bull. Mus. Comp. Zool. Vol. LVI. (Geol. Ser. X) pp. 141-170, and. 
Science, Vol. 32 (1910) p. 723. 


470 


FORMATIONS AND PHYSICAL HISTORY 47t 


Some of the Red Beds in western Texas, New Mexico, and elsewhere 
are perhaps later than Permian, and some in Oklahoma, Kansas, 
Colorado, and perhaps elsewhere, are older. 

In the Staked Plains of Texas the system has its greatest devel- 
opment. The oldest part (Wichita formation) is partly of marine 
and partly of fresh-water origin. The Middle Permian (Clear Fork 
limestone) is of marine origin, and overlaps the Lower. The Upper 
Permian (Double Mountain formation) indicates a reversal of condi- 
tions, for much of Texas was again cut off from the ocean, and con- 
verted into an inland sea or seas, in which the phases of deposition 
common to such bodies of water took place. Occasional beds of 
limestone with marine fossils point to occasional incursions of the 
sea, while deposits of salt and gypsum point with equal clearness 
to its absence, or to restricted connections, and to aridity of climate. 

Throughout much of the area west of the Rocky Mountains 
the Permian has not been differentiated. There is, in places, con- 
formity between the Carboniferous below and the beds classed 
as Trias above, suggesting the presence of unseparated Permian 
between. In northern Arizona and in southwestern Colorado and 
perhaps at other points, there is an unconformity at the top of the 
Permian. The Permian system may have been continuous once 
from’Texas to the Great Basin, by way of New Mexico and Arizona; 
but if so, the continuity of the beds has been interrupted by erosion. 
A very considerable thickness of marine Permian (3,800 feet) is 
reported from Utah. Many Permian deposits of the far west, and 
some of those in the longitude of Texas and Kansas, are red. This 
color characterizes so many formations known to have been made 
in inclosed basins that the connection can hardly be accidental. 

Thickness. In the Appalachian region, the Lower Permian 
beds, sandstone and shale with thin seams of coal, have a thickness 
of about 1,000 feet. The Upper Permian is wanting. In Kansas 
the thickness is twice as great, while in Texas it reaches 7,000 feet. 

Correlation. In the region east of the Mississippi, the Permian 
is so closely associated with the Coal Measures that the two were 
formerly classed together, the Permian being called Upper Barren 
Coal Measures. Were this region only considered, this classifica- 
tion would appear to be satisfactory. In the western part of the 
continent the separation of the Permian from the Carboniferous 
will-probably prove to be more distinct, when details have been 
worked out, and its relation with the Trias close. The Permian 


472 THE PERMIAN PERIOD 


period is best looked upon as a transition period from the Carbonifer- 
ous to the Trias, and so from the Paleozoic to the Mesozoic. Its 
close relationship to the underlying system in some places, and to 
the overlying system in others, is therefore to be expected. 


Foreign Permian 

Europe. In Europe, as in America, the Carboniferous period 
was brought to a close by very considerable changes, for much of 
the area which had been receiving deposits during that period was 
exposed to erosion at its close. Subsequently, much of the same 
surface was again the site of deposition, partly from fresh and partly 
from salt waters. ‘The system is here much more distinct from the 
Carboniferous than in eastern North America. 

In western and central Europe, the Lower Permian (Rothlie- 
gende) consists of a series of clastic formations, together with a large 
amount of igneous rock, in the form of lava-sheets, dikes, and pyro- 
clastic material. The formations and their fossils show that much 
of the sediment was accumulated in inland seas, and in salt and 
fresh lakes. Gypsum, salt, and a meager fauna of dwarfed and 
stunted species are among its distinctive marks. But the sea some- 
times had access to the inland areas of sedimentation, as fossils 
show. The shallow-water or subaérial origin of much of the Per- 
mian is shown by the sun-cracks, rain-pittings, ripple-marks, tracks 
of terrestrial and amphibious animals, etc. In keeping with the 
conditions of its origin the Rothliegende contains some coal. 

Especial interest attaches to the conglomerates and breccias, 
because of their likeness to glacial drift. The conglomerate is wide- 
spread, and in places contains bowlders which have been trans- 
ported great distances; but its glacial origin has not been proved. 

The Upper Permian of western and central Europe (the Zech- 
stein) is unlike the Lower in several important respects. It contains 
much more limestone and dolomite, but neither coal, igneous rock, 
nor, except at its very base, conglomerate. From the stunted 
aspect of the fossils, and from the association of the dolomite with. 
gypsum, salt, etc., it has been thought that the limestone and 
dolomite may be largely chemical precipitates. Some parts of the 
Permian are, however, of marine origin. 

The Upper Permian of central and western Europe contains the 
thickest mass of salt known. Near Berlin, one body of salt has 
been penetrated about 4,000 feet, without reaching its bottom. It 


FORMATIONS AND PHYSICAL HISTORY 473 


may be doubted, however, whether there is a salt Jayer of this 
thickness. Besides common salt, salts of potash and magnesium 
are present locally in such quantity as to be commercially val- 
uable. With the exception of saltpetre, the world’s supply comes 
from these beds. 

The system underlies the larger part of Russia (in Europe), and 
appears at the surface over a large area in the southeastern part of 
















Ai 
i 
Mi 


| 
| 








. Re : ¥ . x i . 
etna be So: 


Fig. 405. Sketch map of Europe during the later part of the Permian period. 
The lines indicate areas of marine deposition, the broken lines areas of lagoon de- 
posits. (After De Lapparent.) 
that country. Here and in southern Europe generally the Permian 
is conformable on the Carboniferous, and is partly marine and 
partly non-marine. In Russia it contains salt, gypsum, etc., and 
also, at some horizons, marine fossils. 

Other continents. In other parts of the world the Permian is 
widely developed. In countries about the Indian Ocean, there is 
a less distinct break between the Carboniferous and Triassic systems 


than in Europe, and locally at least, the Permian seems to bridge 
the interval completely. 


474 THE)PERMIAN! PERIOD’ “' 


Permian glacial formations. The most remarkable fact<about 
the Permian as a whole is that it includes formations of glacial origin, 
and that these occur down to and even within the tropics. Such 
formations are found in all the continents which have large areas in 
low latitudes. 

In Australia, strata of Permian glacial drift are interbedded with 
marine formations and coal beds, the aggregate thickness of the 
whole being not less than 2,000 feet. The recurrence of the bowlder 
beds points to the repeated recurrence of glacial conditions, and the 
great thickness both of clastic beds and of the many included coal 
beds points to the great duration of the period through which the 
several glacial epochs were distributed. 

Counting Tasmania, the glaciation of Australia had a known 
range of nearly 22° in latitude, and about 35° in longitude, though 
it is perhaps not probable that all the area within these limits was 
glaciated. The glacial formations are known chiefly at low levels, 
descending in some places nearly to the sea. Not only is the alti- 
tude of the region low now, but it was probably low during glacia- 
tion, as shown by the interbedding of glacial and marine formations. 

The fossils of the marine beds associated with the glacial 
deposits are similar to those of the Carboniferous (Pennsylvanian) 
period elsewhere, but the plants of the associated coal have a 
Triassic facies. Permian fish remains are found above all the bowl- 
der beds, suggesting that the glacial conditions were over before the 
end of the Permian. The plant fossils therefore indicate that the 
period of glaciation was late Permian or early Triassic; the marine 
fossils, that it was late Carboniferous or early Permian. 

In India, too, there are glacial formations (Talchir conglomerate) 
of about the same age, with fossil plants like those of Australia in 
associated beds. The bed on which the glacial formations rest is 
in some places striated and roche-moutonnéed, as beneath modern 
glacial deposits. These formations are even more remarkable than 
those of Australia, for they reach several degrees: within the Tropic 
of Cancer (to Lat. 18°). Similar formations appear farther north in 
India where marine Permian beds overlie the glacial series. 

In South Africa many of the bowlders of the glacial beds (Dwyka 
conglomerate) are striated, and the bed on which the glacial con- 
glomerate rests shows indisputable marks of ice action in many 
places. The glacial beds are believed to have extended to 26° 4o’. 
In South America glacial conglomerates also are present in the 


LIFE 475 


southern part of Brazil, and in Argentina. The associated coal 
formations carry the same flora (glossopteris flora) as in the other 
continents. 

In the northern hemisphere the glaciation is known to have 
extended from latitude 18° to about 35°, and in the southern, from 
latitude 21° to 35°. In an equatorial zone about 40° in width, 
glaciation has not been discovered. The glaciation can hardly be 
said to be limited in longitude. Glacial conditions must, therefore, 
have prevailed about the borders of an area many times as large as 
that covered by ice in the northern hemisphere during the Pleisto- 
cene glacial period. 

The marked likeness of the floras associated with the glacial 
deposits in these four continents is evidence that there was land 
connection between them at the time of glaciation. The age of the 
glacial beds is not absolutely established, for the Carboniferous 
and Permian are not clearly differentiated in the regions where they 
occur. Perhaps the best judgment that can be formed now is that 
the Paleozoic glaciation culminated in the early part of the Permian 
period. 

Close of the Paleozoic Era 

The close of the Paleozoic era was marked by much more con- 
siderable geographic changes than the close of any period since 
the Proterozoic, though they may be said to have been in progress 
during the Permian period, rather than to have occurred at its close. 
The more important changes in North America, which were far 
advanced by the close of the Paleozoic, were (1) the development of 
the Appalachian mountain system at the western border of Appa- 
lachia; (2) the deformation of the surface of Appalachia; (3) the 
development of the Ouachita Mountains; (4) the final conversion 
of most of the area between the Great Plains and Appalachia from 
an area of deposition to one of erosion; and (5) the restriction of the 
area of sedimentation in the western interior. 

Such extensive geographic changes, involving the conversion of 
extensive areas from sea bottom into land, must have caused pro- 
found changes in the circulation of ocean waters, in the climate of 
many localities, and in the distribution of life. 


LIFE 


The life of the Permian must be interpreted in connection with 
the extraordinary physical conditions which formed its environ- 


476 THE PERMIAN PERIOD 


ment. The salient facts in connection with the physical conditions 
of the period were glaciation and aridity. In view of these facts, 
certain questions relative to the life arise: (1) Did it possess such 
powers of adaptation as to meet its extraordinary environment 
by adjusting itself to it? (2) Was it destroyed co-extensively with 
the changes in environment? (3) Did it elude adverse conditions 
by migrating from one area to another as the adverse conditions 
shifted (hypothetically)? (4) Did its composite experience embrace 
all these alternatives, and if so, what measure of each? 

In the early days of geology it was commonly held that a com- 
plete destruction of all things living on the face of the earth attended 
the close of the Paleozoic era, and that a re-creation followed; for at 
that time, no Paleozoic species was known to have lived on into 
the following era. But it is now known that some species bridged 
the interval, and it is believed that others underwent modifications 
which enabled them to live. The progress of investigation is bring- 
ing more and more evidence of this kind to light. Not only this, 
but the compensating effects of the strenuous conditions in calling 
into play the powers of adaptation and resistance of the organisms 
are coming to be recognized. Notwithstanding all this, it appears 
that the life of the period was greatly impoverished. A census 
made not many years ago gave the known animal species of the 
Carboniferous period as 10,000, while those of the Permian period 
were only 300. A census to-day would probably increase the 
Permian ratio, but the contrast would still be great. 

Plants. The change in the vegetation was rather marked in 
America, though not, at the outset, radical. Of the 107 species of 
plants recorded from the lowest Permian beds of West Virginia and 
Pennsylvania, 22 are found in the Coal Measures below. This and 
other similar facts show that a rather profound change was in 
progress, but that it was not abrupt. But a small part of the total 
floral changes of the Permian appears in the American record, as 
now known; but the nature of the early change is indicated distinctly. 
Lepidodendrons disappeared, Sigillaria became rare, and Calamites 
were greatly reduced. The general features of the fern group re- 
mained much as in the preceding period, but most of the species and 
many of the genera were new. Cordaites continued, and initial 
forms of Ginkgos appeared, giving to the flora a Mesozoic cast. 

In Europe Carboniferous types declined as the period advanced, 
and the general aspect of the flora was that of poverty. Of the new 


LIFE 477 


types which appeared, one is a supposed forerunner of the group to 
which the giant sequoia and the bald cypress belong. 

The most remarkable fact connected with the plant life of the 
period was the evolution of the Glossopteris (tongue-fern), or Gan- 
gamopteris, flora in the southern hemisphere, and its migration into 





Fig. 406. Walchia piniformis, a Permian conifer of Europe. 


the northern. This flora suggests that it was evolved to meet 
the adversities of climate in and about the glaciated regions. De- 
veloped amid adverse surroundings, if not under adverse condi- 
tions, the flora not only took on a resistant aspect in simple outlines 
and compact forms, but gave evidence of its vitality by spreading 
northward into east Africa, Asia, and Europe. It reached northern 
Russia in the later part of the Permian period, and was there asso- 
ciated with forms typical of the European Permian flora. Itis found 
also in Brazil and Argentina. Its vitality is further shown in 
that its descendants became a dominant feature in the Mesozoic 
floras that followed. 

Land animals. Amphibians, which reached their climax in 
the later portion of the Pennsylvanian period, were still abundant 
in the early Permian; but before the end of the period they were over- 
shadowed by the reptiles, which were doubtless their descendants. 

While reptiles’ probably began -to differentiate from amphibians 
earlier, the oldest certain relics of them go back but little beyond the 
beginning of the Permian; but before the close of the period the 
group was large and complex. At least three distinct phyla existed. 
One of them (Pelycosauria), pronouncedly reptilian in character, 


1 Williston, Faynal Relations of Early Vertebrates, Jour, Geol., Vol. XVII, 
1909. 


478 THE PERMIAN PERIOD 





Fig. 407. _ REPRESENTATIVE TYPES OF THE GLOSSOPTERIS FLORA. 4, Glossop- 
teris communis, Fstm.; 6, G. angustifolia, Bgt.; c, Gangamopteris cyclopteroides , 
Fstm.; d, Noeggerathiopsis hislop, Bunb.; e, Neuropteris valida, Fstm.; f, Schizoneur a 
gondwanensis, Fstm.; g, Phyllotheca indica, Bunb.; h, Volizia heterophylla, Bet. 


had branched off before the close of the Pennsylvanian period 
(Fig. 408); another (Cotylosauria) included crawling reptiles with 
large heads, short tails, powerful and short limbs, whose nearest 
and yet rather remote relatives (Pareiasaurus) are found in South 
Africa; the third (Therapsida) included the anomodonts and 
theriodonts, reptiles allied to the pelycosaurs, but more highly spe- 
cialized. ‘The American forms were probably derived from the same 
stock as their African allies, but the types in the two continents 
had, as a result of long isolation, become somewhat distinct. Some 
of the African reptiles are of peculiar interest because of the mamma- 
lian aspect of their skulls, teeth, and other parts of their skeletons. 


LIFE hs . 470 


These were especially abundant in South Africa (Karroo beds!) but 
they have been found also in Europe. The rapid and diverse 
deployment of the early. reptiles in a period of general life-impover- 
ishment is not a little remarkable, but as the reptiles’ were air 





Fig. 408. Paleohatteria longicaudata, from the lower Permian of Germany, 
about % natural size. (Restoration by J. H. McGregor.) 


breathers, the key to their rise may lie in a more oxygenated atmos- 
phere, a point to which we shall return. 

The Permian of Texas and Oklahoma affords the richest Per- 
mian vertebrate fauna now known. In contrast with the verte- 






= Sf ee — —- 
: TAS WDD 
ey i ee 
BELEN 5 SSXiaces 
Ss HY SS 
A, SBN 
uN 
es 
Vs 
Nee 


irae 
| KKK Ale 


S 


CII ATR AFER 
css SS =, 


Fig. 409. Stereosternum tumidum, from Brazil, about 4% natural size. (Restora- 
tion by McGregor.) 


brate fauna of the Pennsylvanian system, this fauna is so unlike 
the Permian faunas of other continents as to imply that land 
animals did not migrate between North America and other conti- 
nents. This isolation seems to have lasted from the later part of the 
preceding period well into the Triassic. 

The Permian record of terrestrial invertebrates is poor. 


1The Karroo beds, so wonderfully rich in significant vertebrate remains, are 
regarded as Permian in part, and Triassic in part. Broom, Geol. of Cape Colony, 


1905, pp. 228-249. 


480 THE PERMIAN PERIOD 


Fresh-water life. Besides the amphibians and some of the 
reptiles which constituted, in a sense, a portion of the fresh-water 
life, fishes were abundant. On the whole they had a rather modern 
aspect. There were fresh-water mollusks, some of which resembled 
unios. 

Marine life. The withdrawal of the epicontinental seas from 
considerable portions of the continents reduced the area available 





D3 Ps. 
Fig. 410. Pareiasaurus serridens, Karroo formation, Cape Coiony, S. Africa; 
about 1/25 natural size. (After Broom.) 


for shallow-water sea life, and so reduced its amount. It is to be 
noted that this reduction came at a time when conditions were 
unfavorable for land life. In North America the restricted marine 
faunas were lineal descendants of ancestors occupying the same 
area. At first, many of the species were the same as those of the 
preceding period, and hence there has always been difficulty in 
drawing a dividing line between the systems. The known species 
of the marine Permian of the Great Plains are only about 70, and 
of these about half are pelecypods. 

The increasing complexity of the sutures of the coiled cephalopods 
has been noted in previous chapters. By the close of the Permian, 
the complexity (Fig. 411), foreshadowed that of the Mesozoic 
ammonites, though older types (goniatites and nautiloids) still 
lived. The ancient straight form (Orthoceras, f, Fig. 411), was in the 
last stage of its long career. The contrast between the disap- 
pearing straight type, in its depauperate form, and the robust youth- 
ful ammonites (a and b, Fig. 411), about to become a ruling 
dynasty, is marked. 


LIFE 481 


Retreatal tracts of marine life. As in previous transition 
epochs when epicontinental waters were largely withdrawn, the 
marine faunas found special refuge in certain embayments or border 
tracts which, in connection with the coastal belts, permitted them 





b 

Fig. 411. MARINE PERMIAN Fossits. a-f, Cephalopods: a, Medlicottia 
copet White; 6 and c, Waagenoceras cumminsi White; d and e, Popanoceras walcotti 
White, three forms of ammonoids with sutures more complicated than in earlier 
forms; f, Orthoceras rushensis McCh., one of the last of the orthoceratites. g-j, 
Pelecypods: g, Pseudomonotis hawni (M. and W.); h, Myalina permiana (Swall.); 
1, Aviculopecten occidentalis (Shum.); 7, Sedgwickia topekaensis (Shum.). k, Mur- 
chisonia sp., a gastropod. 


482 THE PERMIAN PERIOD 


to re-form themselves, regenerate their species, and’ prepare for a 
succeeding invasion of the continental areas. On the American 
continent, the St. Lawrence embayment had done repeated duty 
in this line; but there is no specific evidence that it participated 
notably in the Permo-Triassic transition. The border of the Gulf 
of Mexico, the Mediterranean tract, notably in the region of Sicily 
and southeast Europe, and the Ganges-Indus tract of southern Asia, 

seem to have been special areas of refuge and regeneration at this 
time. Here and on the continental borders generally, the shallow-. 
water marine faunas passed from the Paleozoic to the Mesozoic 
phases. ‘The restriction, compared with the expansional stage of 
the Mississippian period, was great; but the faunas emerged with 
new species born in adversity, ready for conquest when the re- 
advancing seas should give them an expanding realm. Unfortu- 
nately, the sediments in which this transition of faunas should be 
recorded are, for the most part, buried and inaccessible. 


PROBLEMS OF THE PERMIAN 


Between the marvelous deployment of glaciation, a strangely 
dispersed deposition of salt and gypsum, an extraordinary devel- 
opment of red beds, a decided change in terrestrial vegetation, a 
great depletion of marine life, a remarkable shifting of geographic 
outlines, and a pronounced stage of crustal folding, the events of 
the Permian period constitute a climacteric combination. Each 
of these phenomena brings its own unsolved questions, while their 
combination presents a series of problems of great difficulty. These 
marked phenomena were probably related to one another, and their 
explanation is quite sure to be found in a common group of co-opera- 
tive factors. While it is too much to hope for a full explanation at 
once, there is no occasion to blink the facts or evade the issues they 
raise. 

It is to be noted that none of the factors in this combination is 
wholly new to geological history. There had been glaciations al- 
most as strange in early Cambrian times; there had been signs of 
unusual aridity in the salt and gypsum ‘deportes of the Silurian; 
there had been red beds in the Devonian and Keweenawan; there 
had been marked restrictions of life, as at the close of the Ordovician 
there had been extensive weheaeshit changes in earlier Paleozoic 
periods; and there had been foldings of surpassing intensity in 
Archean and Proterozoic times. The peculiarity of the Permian: 


PROBLEMS OF THE PERMIAN 483 


was the complexity of the combination, and the extent of glaciation 
and aridity. 

| The chronological setting of the combination lends some advan- 
tages to its study. It lies in the midst of geologic history, with 
periods of climatic uniformity and polar geniality both before and 
after. No appeal can be taken to a supposed final cooling of the 
earth, or to any senile condition. It was an episode in the midst 
of a long history, and its problems must be faced with this setting 
in mind. 


THE MESOZOIC ERA 


CHAPTER XXII 
THE TRIASSIC PERIOD 


FORMATIONS AND PHYSICAL HISTORY 


When the sea was excluded from the area between the growing 
Appalachians and the Great Plains at the close of the Paleozoic, 
Appalachia appears to have suffered deformation, one result of 
which was the development of elongate troughs upon its surface, 
roughly parallel to the present coast. These troughs became the 
sites of deposition, and the sediments laid down in them constitute 
the only representative of the Triassic system in the eastern part of 
the continent. The open sea seems to have been excluded from the 
western interior by the beginning of the Triassic period, though sedi- 
mentation was in progress over considerable areas between the 
meridians of 100° and 113°. Some of these areas appear to have 
been sites of salt seas and some of fresh lakes, while still others were 
probably without standing water. Between the meridians named, 
many areas of relatively high land probably interrupted the con- 
tinuity of the areas of sedimentation. On the western coast, the 
ocean began to gain on the continent about the close of the Paleozoic, 
and the shore of the Pacific was presently shifted eastward to the 
vicinity of the 117th meridian in the latitude of Nevada. 

In keeping with these changes in geography, Triassic strata are 
known in three regions: (1) The Atlantic slope east of the Appala-. 
chians; (2) the western interior; and (3) the Pacific coast. The 
strata in these three regions are in many ways unlike. 


The Eastern Triassic 


Distribution. The Triassic system of the east occurs in spots 
from Nova Scotia to South Carolina, as shown in Fig. 412. Its 
several areas are mostly elongate in a northeast-southwest direction. 
The beds of these several areas have been grouped under the name 
Newark (Newark, N. J.). 
| 484 





FORMATIONS AND PHYSICAL HISTORY 485 





Fig. 412. Map showing the known distribution of the Triassic system in North 
America (black areas), with conjectures as to its presence where buried (lined areas), 
and its absence where it was once present (dotted areas). 


486 THE TRIASSIC PERIOD 


Kinds of rock. The rocks of this series! include all the common 
varieties of fragmental rocks, some of which are developed in un- 
usual phases. Sandstones and shales predominate, but conglomer- 
ates and breccias are present, and, locally, limestone and coal. 

Conglomerate lies at the base of the system in many places, and 
is made up chiefly of material from the underlying crystalline 
schist. Conglomerates also are the border phase of beds which 
grade laterally into sandstone, and even into shale. The chief 
constituent of the conglomerate is quartz, the most resistant part 
of the underlying terranes; but locally, quartzite, crystalline schist 
and limestone are severally its principal constituents. 

Sandstone and shale make up the great body of the Newark 
series, and both possess distinctive characteristics. The prevalent 
color is red, though there are shales which are black, and sandstones 
which are gray. Some of the sandstone is arkose, that is, contains 
feldspar, and both sandstone and shale contain much mica. Both 
these constituents abound in the metamorphic rocks from which 
the Newark sediments were chiefly derived. Except locally, the 
series is poor in fossils. 

Conditions of origin. The character of the Newark formations 
and their fossils, mainly land plants, footprints of reptiles, and 
fresh- or brackish-water fishes, indicate that they are of continental 
rather than marine origin, but do not tell the precise manner in 


Fig. 413. Diagram showing the development of a trough, now partly filled by 


sediment, by warping. . 




















which they were laid down. The depressions in which these beds 
were deposited may have been due to warping or to faulting, or 
partly to the one and partly to the other (Figs. 413 and 414). How- 
ever formed, they became the sites of lakes, bays, estuaries, dry 
basins, or aggrading rivers. 

The considerable thickness of the sediments, taken in connection 


1 The Connecticut valley and New York-Virginia areas are best known, and 
the descriptions of the formations here given apply especially to them. 


FORMATIONS AND PHYSICAL HISTORY 487 


with their decisive evidences of shallow-water or subaérial origin, 
such as ripple-marks, sun-cracks, tracks of land animals, etc., 
indicate either that the sediments were deposited in an inclined 
position, or that subsidence accompanied the deposition. For the 


\W/; Yf ————————r AY, Yy 


Fig. 414. Diagram showing the development of a trough by faulting. 



























adequate supply of sediment, it would seem that the lands border- 
ing the areas of deposition were raised, relatively, as the troughs 
were filled. 

Former extent. It is possible, and perhaps probable, that the 
areas of the Newark series from Virginia to South Carolina were 
once connected with one another, and with the Pennsylvania-New 
York area, though such connection has not been demonstrated. It 
has even been suggested that the Newark of the New York-Virginia 
areas was once connected with that of the Connecticut valley, and 
this with that of Acadia, the separation being effected by erosion; 
but this suggestion does not seem well founded.! 

Igneous rocks. Igneous rocks are associated with the sedi- 
mentary beds in dikes, and in sheets interbedded with the shales 
and sandstones. Some of the sheets were extruded and subse- 
quently covered by sediment; others were intruded (sz//s) between 
the layers of sedimentary rocks. Certain isolated bodies of igneous 
rock may represent volcanic plugs. The sheets of igneous rock 
(usually called trap, though largely basalt) vary in thickness from 
a few feet to several hundred. 

Structure and thickness. The structure of the Newark series 
is generally monoclinal. In the Connecticut Valley the dip is about 
20° (10° to 25°) to the eastward. In the New York-Virginia area ” 
it is ro°—-15° to the northwest. ‘The strata are otherwise somewhat 
deformed, though never closely folded. The series is faulted ex- 
tensively. On account of the faulting, the thickness of the series 

1 For summary of the Trias of Connecticut, see Davis, 18th Ann. Rept., U. S. 
Geol. Surv., Pt. IT. 


2 For summary of the Newark of New York and New Jersey, see Kiimmel, 
Rept. of the State Geologist of New Jersey, 1896, and Jour. Geol., Vol. VII. 


488 THE TRIASSIC PERIOD 


is not easily determined. In the Richmond area of Virginia, it is 
estimated at something more than 3,000 feet; in New England, at 
7,000 to 10,000 feet; and in New Jersey even more. 


CORN WA LLIS HILL VINITA TUCKAHOE CREEK 





w. 
Sar 
Mo 


pig MT 
EG 


f\ 
Wiig ap sip 
ay 
hy y « yy H 
Gy 
Fig. 415. Structure of the Newark series on the James River, Richmond area, 


Va. AA, minor flexures; ff, faults. Structure of the deeper parts hypothetical. 
The heavy black band represents coal. (Shaler and Woodworth, U.S. Geol. Surv.) 


Correlation. The stratigraphic relations of the Newark series in 
the United States would not determine its age. It lies unconfor- 
mably on rock which is mainly pre-Cambrian, and is overlain un- 
conformably by Comanchean (Lower Cretaceous) beds. About 
the Bay of Fundy, however, the rocks are unconformable on the 
early Permian. The physical relations of the Newark series there- 
fore show that it is post-early-Permian, and pre-Comanchean. 
In referring the series to the Triassic, the chief reliance is on the 
fossils, and on the same basis it is believed to represent only the 
later part of the period. 


The Western Triassic 


The western interior.' The interior area of sedimentation, 
chiefly between the tooth and 113th meridians, had its southern 
limit, so far as now known, near the southern boundary of the 
United States, while at the north it extended into Canada. This 
area is believed to have been cut off from the Gulf by land in eastern 
Texas. Into this interior area of sedimentation, detritus was borne 
from the surrounding lands. The conditions of sedimentation were 
much as in the Permian period. The structure of some of the sand- 
stone is such as to suggest an eolian origin. 

The deposits of the period are largely concealed by later beds, 
but they are exposed at various points where the strata have been 
warped, and the overlying beds removed by erosion. The most 
easterly outcrops are in Texas, Oklahoma, and South Dakota, and 


1 There is some doubt about the age of some of the beds formerly referred to 
this system. The tendency of later study has been to refer more and more of 
them to the Permian. 


FORMATIONS AND PHYSICAL HISTORY 480 


red beds which are thought to be Triassic outcrop interruptedly 
along the eastern base of the Rocky Mountains from British America 
to New Mexico. These beds are thin, and contain more or less 
gypsum and salt. Here and there they contain fossil leaves. 

Farther west, red beds have representation among the surface 
rocks, and some of them are perhaps Triassic; but in much of the 
western interior, undifferentiated Triassic and Permian rest con- 
formably on Carboniferous (Pennsylvanian). In southwestern 
Colorado and eastern Utah, the Trias is unconformable on older, 
deformed, unfossiliferous red beds (presumably Permian), and on 
strata of Pennsylvanian age.! 

In the eastern part of the western area, the Triassic system is 
thin, in places no more than too feet. To the west it thickens, 
reaching 2,000 to 2,500 feet in the Uinta Mountains, beyond which 
it again becomes thinner. 

The Pacific slope. The Triassic system has here its greatest 
development in America. In the latitude of Nevada, the Pacific 





Fig. 416. Chugwater (Triassic) Red Beds near Shell, Wyo. (U.S. Geol. Surv.) 


seems to have extended eastward over the site of the Sierras to 

longitude 117° (approximately). Farther north the shore line has 

not been located definitely. It probably was irregular, and, in 
1 Cross and Howe; Bull. G. S. A., Vol. xvi, p. 447. 


490 THE TRIASSIC PERIOD 


general, several degrees farther east than now, well up into British 
Columbia. Between the latitudes of 55° and 60’, the sea is believed 
to have crossed the present Cordilleran belt. 

The published measurements assign the system the great thick- 
ness of 17,000 feet (maximum) in the West Humboldt range of 
Nevada, where it rests on pre-Cambrian terranes. To have sup- 
plied such a volume of sediment, the land to the east must have 
been high, or repeatedly renewed, unless the great thickness is due 
to oblique deposition. 

Climatic Conditions 

The wide distribution of gypsum and salt in the system, in more 
than one continent, is evidence of wide-spread aridity. The prev- 
alent redness of the system, in other continents as well as our own, 
is also commonly regarded as an indication of aridity. Perhaps the 
peculiarities of the Newark conglomerate may find their explanation 
in such a climate, which favors great changes of temperature, and 
so the disrupting of rock, if it is not covered by soil. Under such 
circumstances, much coarse debris originates, largely of rock which 
is undecomposed. Violent storms (cloud-bursts), which character- 
ize some arid climates, might account for the transportation of 
coarse debris from its place of origin to its site of deposition. For 
the formation of abundant debris in this way, steep slopes are 
needful, for gentle slopes and flats soon get a covering of mantle 
rock which prevents the disruption of the rock beneath. If this was 
the origin of the coarse materials of the conglomerate, their rounding 
and wear would have to be attributed to the waves or currents of 
the water in which deposition took place. 


Close of the Trias 


Considerable geographic changes marked the close of the Trias- 
sic period in eastern North America, bringing the areas which had 
been the sites of deposition to higher levels, faulting the rocks, and 
affecting them by igneous intrusions. In the western part of the 
United States, the separation of the Triassic period from the Jurassic 
was not pronounced, and the sedimentary history of much of this 
part of the continent seems to have run an uninterrupted course 
from the beginning of the Permian to the later part of the Jurassic. 
The case may have been somewhat different north of the United 
States, for in British Columbia and in the adjacent islands, Triassic 
and older formations were upturned, deeply eroded, and again sub- 


FORMATIONS AND PHYSICAL HISTORY 491 


merged before the beginning of the Cretaceous. The great igneous 
formations associated with the Trias of the northwest appear to 
have been made during the Triassic period, rather than at its close. 


Foreign Triassic 


Europe. In Europe, the Trias is exposed in many widely sep- 
arated areas, the largest being in northwestern Russia; but the 
system is better known in the western part of the continent. In 
England, it is unconformable on the Permian, but on the continent, 
generally conformable. It has a marine and a non-marine phase. 
The non-marine (or Triassic) phase prevails throughout the north- 
ern part of the continent, while the marine (or Alpine) phase is 
found farther south. ‘The former resembles the Permian of Europe, 
and the Permian and Triassic of the United States. 

In general, the Upper Trias is more widespread than the Lower, 
especially in the southern part of the continent, and is marine over 
a wider area. The principal subdivisions recognized in Britain 
and Germany are the following:— 


Britain Germany 
Rhaetic Keuper 
Upper Trias Muschelkalk 
Lower Trias Bunter 


In Germany, the middle member is largely marine, and the others 
chiefly non-marine. In England the system is often known as the 
New Red Sandstone, though formerly the Permian was also included 
under this term. It differs from the Trias of Germany chiefly in 
the absence of the marine member. Both salt and gypsum occur 
in workable quantities in some parts of England. The Triassic 
beds of most of Russia are similar to those of western Europe. In 
southern Sweden, the system contains coal. 

The non-marine formations of red color, so characteristic of the 
Triassic system both in North America and Europe, afford another 
striking intercontinental analogy, and doubtless point to a common 
cause, or to similar widespread conditions. 

The Alpine or marine phase of the Triassic has its best develop- 
ment in the eastern and southern Alps, and is made up of thick beds 
of limestone and dolomite, alternating with thinner beds of clastic 
rock. ‘The limestone and dolomite are much more resistant than 
the associated shales, and, as a result, erosion has developed a dis- 
tinctive topography (known as “‘the dolomites’’) at several points in 


492 THE TRIASSIC PERIOD 


———— 


sl 
ji 


at if 


e i. 

















| Br FG 





Fig. 417. Sketch-map of Europe showing areas of sedimentation in the early 
part of the Triassic period. The broken lines represent areas of non-marine de- 
posits; the full lines, areas of marine deposits. (After De Lapparent.) 


the southern Alps—a topography so striking that the localities where 
it is seen have become the objective point of travel, both for geolo- 
gists, and for lovers of wild and picturesque scenery. In these re- 
gions the dolomite (limestone) stands up in bare, bold-faced walls, 
peaks, and towers, surrounded and separated by valleys and passes 
clothed with abundant vegetation. The decay of the projecting 
limestone leaves little soil behind, and the little formed is promptly 
carried away by wind and rain. The Trias of the western Alps is 
largely non-marine, and in some parts of Switzerland the Upper 
Trias contains coal and igneous rocks. The Trias of the Italian 
Alps is the source of the Carrara marble. 

Other continents. The marine phase of the system, similar to 
that of southern Europe, continues eastward to southern Asia. It 
is found also in the high latitudes of Asia, including numerous 


LIFE 493 


islands ncrth of the mainland. The Trias here is generally con- 
formable on the Permian and beneath the Jurassic. 

In South America no marine deposits of Triassic age are known 
east of the Andes, but coal-bearing Trias occurs in Argentina and 
Chile, and marine beds at various points in the Andes. Thus it 
is clear that the site of parts of this great ere of mountains was 
beneath the sea in the Triassic period. 

The Triassic system is represented also in South Africa, Aus- 
tralia, New Zealand, and New Caledonia. 


LIFE 


The remarkable physical conditions that impoverished the land 
life of the Permian period held sway during the early part of the 
Triassic, and the two periods were much alike in their general 
biological aspects, as in their physical conditions; but toward the 
close of this period there was a pronounced change. The land 
became lower and the sea encroached upon it, bringing about appro- 
priate changes in life. Nearly all that is known of life in North 
America belongs to the later portion of the period. 

Plants. Plant life was probably meager, for broad saline basins 
and arid tracts are inhospitable to it. At any rate, its record is 
scanty. ‘The Triassic was distinctly an age of gymnosperms. Ferns 
and fern-like plants were still important but their dominance was 
past. The great lycopods, too, were almost gone, though sigillarias 
were among their lingering representatives. Calamites had given 
place to true equiseta, which were represented by gigantic forms. 
Among gymnosperms, cordaites had declined, and ginkgos (Gink- 
goales) diverged from them at about this time. Conifers of the 
types that came in during the Permian, and kindred new ones, were 
prominent. The cycadean group (p. 685) occupied the place of 
central interest. The Bennettitales (p. 685), formerly called cycads, 
abounded, and from them the true cycads sprang. The Triassic 
floras of Europe and America, so far as known, were much alike. 
Both had a scrawny pauperitic aspect that reflected the hostile con- 
ditions in which they lived. In the far east and in the southern 
hemisphere, the genus Glossopteris and its allies constituted a marked 
feature of a flora whose general aspect was much like that of the 
preceding Permian flora in the same regions. 

In the closing stages of the period an ampler flora seems to record 
some amelioration of the inhospitable conditions. The larger part 


494 THE TRIASSIC PERIOD 


of the known American fossils belong to this stage. The Richmond 
coal-beds of the Newark series, probably the product of marsh 
vegetation, contain great numbers of equiseta and ferns, but almost 
no conifers and few cycadeans. 

Land animals. The physical conditions of the Permian and 
Triassic periods were so similar that adaptation to the conditions of 
the first would seem to have been a fitting preparation for life in the 
second. Yet, in spite of this fact, there was a great break in the 
succession of land life, so far as the known record shows. What — 
became of the Permian vertebrate faunas of North America is un- 
known, for between the horizons yielding Permian fossils of land 
animals, and those yielding Upper Triassic fossils of land animals, 
there are great thicknesses of red sandstone barren of fossils of all 
sorts, so far as now known, and the later fauna does not appear to 
have descended from the Permian. In Africa there appears to have 
been a much less serious break between the land life of the two 
periods. In other continents few Early and Middle Triassic fossils 
of land life have been found, but the life of the Upper Trias is better 
known. During the period many types were initiated, while only a 
few reached their maximum development. 

There is abundant proof of the mingling of European and 
American land faunas late in the period, for at this time there were, 
in North America, representatives of groups that had lived in 
Europe since the early Permian, but which had never before ap- 
peared in our continent, so far as now known. 

Though still numerous the amphibians had lost the foremost 
place they held in the Permian. Before the close of the period they 
entered upon a rapid decline from which they never recovered. 
Ancestors of the whole tribe of terrestrial vertebrates, they soon 
became its most insignificant representatives. 

Reptiles evolved rapidly. The branch with the mammalian 
strain (p. 478, Fig. 418) seems to have been left far behind by the 
more distinctively reptilian branch, which developed greatly later 
in the period when the dryness was ameliorated and vegetation began 
again to flourish. Before the close of the period, every important 
group of the class was represented. Crocodilians, flying saurians, 
and the scaled reptiles (lizards, snakes, etc.) came in near the close 
of the period, as some of the older types were disappearing. 

A foremost feature of the life was the advent and rapid evolu- 
tion of the dinosaurs (terrible saurians). At first they were of 


LIFE 495 


generalized types, but later became more specialized, and widely 
divergent. While some were small and delicate in structure, others 
were gigantic and ungainly. Carnivorous forms only (Theropoda) 





Fig. 418. Oudenodon trigoniceps; an anomodont (or dicynodont) from the 
Karroo of South Africa, similar to forms of the Trias in Wyoming. (After Broom.) 


are known in the Trias, and most of them were not especially large. 
Their general form is indicated by the partially restored skeleton 
shown in Fig. 419. The strength of the hind parts, the relative 





fig. 419. A Triassic dinosaur of the Connecticut Valley, Anchisaurus colurus; 
ressored by Marsh; 1/30 natural size, 


OO THE TRIASSIC PERIOD 


weakness of the fore limbs, and the kangaroo-like attitude, are the 
most obvious features. The bones of the upright-walking forms 
were hollow, and some other structural features resemble those of 
birds. The reduction of the toes of the hind feet to four, with one 
of them much shorter than the others, caused their three-toed tracks 
to be mistaken for those of birds, until recently. The dinosaurs 
had wide range, living in the Rocky Mountains, along the Atlantic 
coast from Carolina to Prince Edward Island, in western Europe, 
India, and South Africa. : 

Before the close of the period the reptilian tribe sent delegations 

xs 


am ns, 





te ai Wi aaa 
BS = 


Fig. 420. A Triassic sauropterygian, Lariosaurus balsami, restored; about 1/11 
natural size; from the Muschelkalk, Lombardy, Italy. (After Woodward.) 


to sea (Fig. 420), but marine forms were more plentiful in the next 
period. 

Advent of mammals. Of especial interest is the appearance of 
early form of mammals. They were small, and so primitive in 
type that it is not altogether certain that they were mammals; but 
they are commonly regarded as such, with kinship to the marsupials. 
Their appearance while reptiles were yet dominant suggests that 
mammals diverged from the primitive stock much earlier. In view 
of the mammalian dominance of later times, it is noteworthy that 
they developed but slowly and feebly during the Mesozoic era. 

Marine life. Except along the Pacific coast, there is, in North 
America, little record of the marine life of the Triassic period; but 
in Europe the record is better. While the sea withdrew from the 
northwestern part of Europe during the Permian period, it lingered 
about the Mediterranean, in Russia, Turkestan, and northwestern 
India, and probably on the continental platform in or near Siberia. 
The Mediterranean, the Himalayan, and the Siberian regions are 
the best known tracts into which the shallow-water marine life of 


LIFE 407 


the late Paleozoic retreated and gave rise to the early provincial 
faunas of the Mesozoic. 

In each of these three areas an important remnant of Paleozoic 
sea life seems to have undergone a radical and perhaps rapid evolu- 
tion, such as might be anticipated from the crowding of the great 
faunas of earlier times into limited areas. From these areas the 
new faunas spread when the sea again extended itself upon the land. 

The most complete record of the transition from Paleozoic to 
Mesozoic marine life is found in India. Beds containing fossils 
characteristic of the Permian are overlain conformably by beds con- 
taining forms characteristic of the Mesozoic. In the Permian beds 
there are forms foreshadowing the Mesozoic types, and in the beds 
above there are Permian types that lived on and mingled with 
Mesozoic forms. The transition fauna of the Mediterranean region 
appears to have been less rich. Concerning the early stages of the 
Siberian fauna, little is known; but its peculiarities, as revealed in a 
later stage of the early Trias, leave little doubt of its independence 
of origin. ; 

It is quite certain that there was at least one other area where 
important faunal reorganization took place, for a notable fauna 
appeared suddenly in the Middle Triassic, which does not seem to 
have originated in any of these three districts. 

Geographic suggestions of the faunas. The alliance of the Indian 
forms with those of North America is so close as to indicate that 
before the close of the early Trias, migratory connections had been 
established between India and western America. 

Somewhat later in the early Trias there appeared in the Siberian 
region (Olenek River) a fauna having some of the same genera as 
the Indian. Closely related species are found in Idaho. If there 
was connection between the Indian and Siberian regions, it would 
be possible for Indian species to reach America from Siberia either 
by way of the Arctic coast, or by the Pacific sea-shelf, and slight 
changes, involving submergence or emergence in the region of Bering 
Strait might change the combination of the faunas. 

The Indian and Siberian provinces seem to have been distinct 
from the Mediterranean province throughout the earlier Triassic; 
but in California a few fossils have been found which are character- 
istic of the earlier Triassic of southern Europe. 

The early Triassic faunas of central Europe were very diverse, 
a part being developed apparently in fresh water, a part in isolated 


498 THE TRIASSIC PERIOD 


seas, and a part perhaps in gulfs and bays. The marine life was 
scanty, and its origin and relations uncertain; but it seems to have 
been largely independent of the Mediterranean basin. 





Fig. 421. A Group oF Triassic CepHatopops. a, Trachyceras austriacum 
Mojs.; b-c, Tropites subbullatus Hauer; d, Choristoceras marshi Hauer; e-f, Ceratites 
nodosus de Haan, lateral and ventral views of the shell. 


LIFE 499 


By the middle of the Triassic period the faunas had begun to 
intermingle, and to lose their provincial characteristics. The Med- 
iterranean fauna gained access to the Indian basin and to our 
western coast, and counter-migrations were of course made possible. 
At about the same time, the Siberian fauna had access to western 
United States. — 

During the later stages of the period a rich marine fauna 
flourished in California. Many of its species were identical with 
those of the Mediterranean and Himalayan regions, or closely allied 
to them. It is therefore inferred that these provinces were in free 
communication, so far as marine life was concerned, with the west 
American coast. 

Prominent types. The most conspicuous feature of the Triassic 
faunas was the re-ascendancy of the cephalopods in the form of the 
ammonites (Fig. 421), which had a marvelous development during 
the period, reaching a thousand species. Their evolution was the 
more notable because the structural changes were conspicuous, and 
showed plainly the advance of each stage over the preceding. While 
early types still persisted, closely coiled, intricately-sutured forms 
predominated. ‘The first representatives of the cuttlefish type 
appeared at this time. The deployment of the cephalopods was 
therefore greater than ever before, though they did not reach their 
culmination till the next period. Old forms, orthoceratites and 
goniatites, made their last appearance in this period. The re- 
markable commingling of old and new types makes this one of the 
most instructive assemblages in the history of the cephalopods. 

A similar commingling of transitional forms was presented by the 
gastropods, and the progress of the bivalves was scarcely less real, 
though they do not show the transition from ancient to modern so 
conspicuously. ‘Their numbers were large, and most of their genera 
modern, some being identical with those now living. With the 
modern types there were about half as many that still bore a Paleo- 
zoic aspect. 

The dominant brachiopod types of the late Paleozoic were dis- 
tinguished by extended hinge-lines, while the narrower beaked or 
rostrate forms were in a respectable minority. In the Triassic 
period the latter became predominant, and have remained so ever 
since. (Compare Figs. 422 and 367.) 

Among echinoderms, leadership passes from the crinoids to the sea- 
urchins. Starfishes and brittle-stars were present, but not abundant. 


500 THE TRIASSIC PERIOD 


Corals were rare in most places, but abundant in favored lo- 
calities. Some of them resembled Paleozoic forms in being simple 
and cup-shaped, but compound species took on the modern (hexa- 
coralla) form, and the compound Paleozoic (tetracoralla) type 





i 

Fig. 422. Group oF Marine Triassic Fosstts. a, b, and c, cephalopods: a 
Ceratites whitneyi Gabb; b, Orthoceras blakeit Gabb; c, Meekoceras. d-g, pelecypods: 
d, Corbula blaket Gabb; e, Myopharia alta Gabb; f, Myacites humboldtensis Gabb; 


g, Pecten humboldtensis Gabb; h and i, brachiopods: h, Rhynchonella equiplicata 
Gabb; 7, Terebratula deformis Gabb. 


disappeared. These later compound corals do not seem to have 
descended from the compound Paleozoic forms, but from some simple 
type. 

Though the general aspect of the Triassic marine faunas was 
revolutionary, it was yet transitional, and not a new fauna substi- 
tuted for an old one. Paleozoic types lived side by side with later 
forms, though in most cases represented by new genera. ‘This over- 
lapping and commingling of old and new indicates clearly the grada- 
tion of the earlier into the later. The transition was extraordinary 
in the apparent rapidity of its progress, and in the extent to which it 
affected all classes. The fact that most of the new types lived 
at the beginning of the Triassic indicates that the transition was 
chiefly in the Permian. The fundamental cause was, with little 
doubt, the readjustment of the earth’s surface to internal stresses, 
and the physiographic and climatic changes consequent upon this 

readjustment. 


LIFE £6i 


Marine reptiles seem to have thriven on the western coast of 
our country, especially in the middle and later Trias. The numer- 
ous ichthyosaurs found in the later Triassic beds of this region 
suggest that it may have been a center cf dispersion of these rep- 
tiles. With the ichthyosaurs were other reptiles (thalattosaurs) 
unknown elsewhere. 


CHAPTER XXIII 
THE JURASSIC PERIOD 


FORMATIONS AND PHYSICAL HISTORY | 

Eastern North America. Jurassic formations have not been 
identified in the eastern half of the continent, where erosion seems 
to have been the leading geologic process during the period. Its 
effectiveness may be judged by the fact that both the uplifted and 


if 


= 


i rae eet 





Fig. 423. Map showing the general 
relations of land and water in the western 
part of North America during the later 
part of the Jurassic period. The black 
areas represent known areas of Upper 
Jurassic. The dotted line is the conjec- 
tured outline of the bay. (After W. N. 
Logan.) 


502 


deformed Triassic system and 
the Appalachian mountain re- 
gion farther west were essen- 
tially base-leveled before the 
next period was far advanced. 

Western interior. Deposi- 
tion was in progress, probably, 
in some parts of the western 
interior, though early Jurassic 
beds of this region have been 
clearly differentiated from the 
Trias in but few places. There 
is perhaps room for doubt 
whether the lower and middle 
parts of the system have much 
representation in this region. . 

Late in the period, an arm 
of the sea covered a large tract 
in the western interior, cover- 
ing much of Wyoming, Mon- 
tana, Utah, and Colorado, and 
parts of several other states 
(Figs. 423 and 424). This is 
shown by the presence in these 
states of sedimentary beds con- 
taining marine fossils of late 
Jurassic age. The avenue 


FORMATIONS AND PHYSICAL HISTORY 503 


ett tae et 


‘ 
: 
‘ 
‘ 

4 
r 
‘ 
' 

' 
’ 
' 





Fig. 424. Map showing the areas where the Jurassic system appears at the 
surface in North America. The conventions are the same as on preceding maps. 


through which the sea entered has not been determined, but the 
fossils of the interior are so unlike those of California, and so like 


504 THE JURASSIC: PERIOD 


those of the Queen Charlotte Islands and British Columbia, as to 
suggest that the waters entered from the northwest (Fig. 423). 
The presence in some parts of the western interior, of fresh-water 
beds (Morrison beds of Colorado, Montana, and Wyoming) regarded 
by some as of late Jurassic age, would, were their age established, 





Fig. 425. Lower part of the LaPlata (Jurassic) sandstone, southwest of La Sal 
Mountains, Utah. (Cross, U. S. Geol. Surv.) 


show that the sea-water withdrew before the end of the period. If 
not Jurassic, the beds in question are Comanchean. | 

Marine Jurassic limestone occurs in western Texas. Its con- 
nections are probably southward with the Jurassic of Mexico, where 
the system is well developed. | 

Pacific coast. Marine deposition was in progress on the Pacific 
coast, though much of the system here is concealed beneath younger 
formations. In the latitude of Nevada and Utah, the earlier 





Fig. 426. A section in southern Montana. A®=Archean; €, Cambrian; D, 
Devonian; Mm, Mississippian; Pg, Pennsylvanian; Je, Jurassic; Kd, Kmc, and 
Kl, Cretaceous; bbr, igneous rock. (Peale, U. S. Geol. Surv.) 


formations of the period extended east to longitude 117°. The 
Lower Jurassic beds generally rest on the Trias conformably where 


FORMATIONS AND PHYSICAL HISTORY 505 


both are present, but the younger beds overlap the older system at 
some points, and fall short of it at others. 

The system contains the common sorts of sedimentary rocks, 
and some fragmental igneous rock. Jurassic formations also are 


Tc Nt ; 


ANNES N 





at xX a ere 
‘ \ es on Ns s tN 4 oe . toe alF co \N \ i \ 
Dh ee we, ah 


ROACANALEAG 

Fig. 427. Section in the Sierras of California. The Jurassic (or Jura-Trias) 
has been metamorphosed, and is associated with igneous rock. grd and dpt, igneous 
rock, probably of Jurassic or Cretaceous age; s/ and s/m, Jura-Trias (?) schist; Na, 
Nr, and Pb, igneous rock, late Tertiary and Pleistocene. (Lindgren, U. S. Geol. 
Surv.) 
known at somewhat widely separated points in Alaska.'. On the 
shores of Cook Inlet, 10,000 feet of Middle and Upper Jurassic are 
reported. 

Thickness. The total thickness of the system in California does 
not. exceed 2,000 feet (in part tuff). Farther east, in western 
Nevada, nearer the land whence sediment was derived, it attains 
a thickness twice or thrice as great. In the western interior, it is thin. 

Surface distribution and position of beds. ‘The Jurassic beds do 
not now appear at the surface over large areas, being much con- 
cealed by younger beds. In some areas they retain their original 
position, while in others they have been tilted, or even folded or 
metamorphosed (Fig. 427). This is especially the case in the 
Sierra Mountains and in some ranges near the western coast. 


Close of the Period 


Orogenic movements. At the close of the Jurassic period, there 
were considerable disturbances in the western part of North Amer- 
ica. Great thicknesses of Triassic and Jurassic strata began to be 
folded into the Sierras, and the Cascade and Klamath mountains 
farther north perhaps began their growth. It is not to be under- 
stood that these mountains attained great height at this time, or 
that they have not had later periods of growth. It is probable that 
the Coast Range of California began its history at the close of this 
period, for deformed Jurassic beds (Golden Gate series). underlie 

1 See Alaskan Reports, U. S. Geol. Surv. 


506 THE JURASSIC PERIOD 


the Lower Cretaceous unconformably in the axis of the range; but 
the movements which gave the Coast Range its present form 
(modified by erosion), took place much later. Various other ranges 
of the west are thought to have begun their history as mountains 
at about the same time. After this closing-Jurassic period of 
orogenic movement, the coast was somewhat farther west than now 
in northern California and southern Oregon. 

Toward the close of the period, much, if not all, of the great 
Upper Jurassic gulf of the northwestern part of the continent dis- 
appeared. All in all, the deformations at this time were greater 
than those which mark the close of most periods. 


Foreign Jurassic 


Europe. Jurassic strata are exposed in many and widely sepa- 
rated parts of Europe, though for the most part in small areas only. 
It has been thought that the Jurassic of England is probably con- 
tinuous with that of France beneath the English Channel, and 
thence, by way of southeastern France, with those parts of the 
system which appear about the Mediterranean, and by way of 
Belgium, the Netherlands, and the German lowlands, with those 
parts which appear in Poland and Russia. The lower part of the 
system (Lias) is less extensive than the Middle, and the Middle 
less widespread than the Upper. Progressive submergence was, 
indeed, one of the features of the period. 

Among the more distinctive features of the system in Europe are 
the following: (1) A considerable content of coal in some places, 
notably Hungary. (2) The abundance of odlitic limestone, both in 
England and on the continent. (3) The presence of lithographic 
stone (Solenhofen limestone of southern Germany). This stone is 
so fine and so even-grained, and at the same time so workable and 
so strong, that it has come into use the world over for lithographic 
purposes. The stone is also remarkable for the perfection of its 
fossils, including such delicate parts as the gauzy wings of insects. 
(4) The considerable development of non-marine beds in the lower 
part of the system, and again at its very top. 

The close of the period in Europe was marked by a somewhat 
widespread emergence of land. In central Europe, the emergence 
began before the close of the Jurassic, for the latest beds (Purbeck) 
of the system in England are unconformable on beds lower in the 
system. Similar changes are known to have occurred in late 


FORMATIONS AND PHYSICAL HISTORY 507 


Jurassic time in some other regions. On the other hand, the Upper 
Jurassic and the Lower Cretaceous beds are in places so closely 
associated as to show that no change of continental dimensions 
brought the Jurassic period to a close. 

Other continents. The Upper Jurassic is widespread in Arctic 
lands. This points to a great Arctic sea in the later part of the 
period, with two considerable dependencies to the south — the one 
in Russia, the other in western America. The Lower Jura is want- 
ing in these latitudes, so far as known,‘and the Middle Jura is limited. 
The Lower Jura occurs in southwestern Asia and Japan. The 
Middle Jura, largely clastic and of terrestrial origin, is widespread 
in Northern Asia, and marine Middle Jura is known in northern 
India. The Upper Jura is much more extended, especially in the 
north. The system is known in New Zealand, Borneo, Australia, 
and South America (Peru, the Bolivian Andes, Chile, and Argentina). 

Coal. Coal is somewhat widely distributed, occurring in Hun- 
gary, the Caucasian region, Persia, Turkestan, southern Siberia, 
China, Japan, and Farther India, in many of the islands southeast 
of Asia, in Australia and New Zealand. Most of the coal is in the 
lower part of the system (Lias). Outside of North America, it is 
probable that no other system except the Pennsylvanian contains 
so much coal. 

Climate 

The testimony of fossils gathered in various parts of the world 
is to the effect that the climate of the Jurassic period was genial. 
In Europe, corals lived 3,000 miles north of their present. limit, 
and saurians and ammonites flourished within the Arctic circle. 
Nevertheless, climatic zones probably were defined. The detailed 
study of the faunas has led to the belief that one climatic zone is 
recorded in the Jurassic beds of the Arctic belt, a second in the 
deposits of central Europe, and a third in the southern province of 
Europe and lands farther south. 


LIFE 
The Jurassic was a period of sea extension, and the marine life 
again assumes a place of leading importance in the fossil record. 
At the saine time the land life, though suffering somewhat by the 
smaller area available for it, was favored by genial climate. 
Marine life. The faunal progress of the period is less well 
revealed in North America than in Europe and Asia, The great 


508 | THE JURASSIC PERIOD 


features were (1) the continued dominance of ammonites 
among the invertebrates, (2) the rise of the belemnites, (3). the 
abundance and modernization of pelecypods, (4) the rejuvenation 
of corals and crinoids, (5) the marked development of sea-urchins, 
(6) the introduction of crabs and modern types of crustaceans, (7) 
the prevalence of foraminifera, radiolaria, and sponges, (8) the 





Fig. 428. Group oF JurAsstc AmMonitTES. a-b, Coroniceras bisulcatum 
(Brug.), a lateral and ventral view of one of the Arietide; c, Deroceras subarmatum 
(Young); d, Perisphinctes tiziant (Oppel); e, Reineckia brancoi Steinm. 


LIFE 509 


change in the aspect of the fishes, and (9) the great sea-serpents, 
descended from land-reptiles. 

(1) The ammonites were represented by many beautiful forms 
(Fig. 428). They deployed along ascending lines in most cases, but 
erratic and degenerate tendencies showed themselves. Despite 
these adverse foreshadowings, the ammonities were still in the 
heyday of their luxuriance and beauty. 

(2) Another division of cephalopods, the belemnites, had appeared 
in the Trias, and rose to prominence rapidly. They are represented 
in the fossil state chiefly by their internal shell or ‘‘pen” (Fig. 429). 


=> 


Fig. 429. The internal shell of a belemnite, restored; the lower, solid, conical 
portion (at the left in the Fig.), the part most commonly preserved, is the rostrum 
or guard; the middle portion is the phragmocone, which is a diminutive chambered 
shell with septa, siphuncle, and protoconch as in the older tetrabranch order; the 
upper part is the prostracum, which corresponds to the ‘‘pen”’ of living cuttle-fish. 





In the course of the period the belemnites came almost to rival the 
ammonites, and were almost as characteristic of the successive 
stages of deposition. The first known cuttle-fishes also appeared 
at this time. 

(3) Pelecypods flourished during the period (Fig. 430), and, took 
on a markedly modern aspect, the oyster family taking the lead. 
Gastropods were abundant in some places, but singularly absent 
in others. Existing genera were represented. 

(4) Suggestive of shallow clear seas was the reappearance of 
corals and crinoids in abundance in the later part of the period. 
The modern type of corals ( Hexacoralla) was in the ascendant and 
formed reefs, especially in European seas. Crinoids also rose again 
to prominence, though their diversity was not great. Most of them 
lived in shallow water, as most of the Paleozoic types had; but there 
is evidence that deep-water species had appeared, leading toward 
the prevalent habit of the present. 

(5) The slow evolution of the sea-urchins in the Paleozoic era 
was succeeded in the late Trias by the beginning of a rapid evolution, 
which reached its climax in the early Tertiary. 


510 THE JURASSIC PERIOD 





Fig. 430. Group OF Jurassic PELECyPops. 4d, T'rigonia navis Lam.; b, Gry- 
phea arcuata Lam.; c, Ostrea deltoidea Sby.; d, Exegyra (Ostrea) virgula D’Orb.; e, 
Aucella mosquensis Keys. 


(6) The trilobites of the sea, and the eurypterids of land waters, 
had been succeeded by decapods which rose to a moderate and pro- 
longed ascendancy. The prawns and lobsters (Macrura, long- 
tailed decapods) were the earlier division, and the most numerous in 
this period; but the first known crabs (Brachyura, short-tailed 
decapods) appeared before the period was past. ‘The macrurans 
seem to have frequented embayments and protected locations near 
the land, or perhaps within it, for terrestrial, fresh-water, and marine 
species are preserved in the same sediments. Probably macrurans 
had representatives in terrestrial waters then, as now. 

(7) Sponges and foraminifera abounded and are well preserved. 

(8) The marked change in the aspect of the fishes which set in 
during the Trias was carried farther in this period. Some of the 
older types declined; but the selachians (sharks) remained abundant, 





Fig. 431. JURASSIC C@ELENTERATA AND ECHINODERMATA. a and 6b, Tham- 
nastrea prolifera Becker, a complete corallum, and the lateral surface of a costal 
septum, enlarged; c, Thecosmilia trichotoma (Goldf.); d, Pentacrinus briareus Mill; 
e, Cidaris coronata Goldf. : 


skates and rays began their modern career; the existing family 
(Chimeride) of sea-cats or spook-fishes made its appearance, so 
far as fossils show (Fig. 434); the forebears of the living garpikes 


512 THE JURASSIC PERIOD 





Fig. 432. Jurassic Fossirs. a-c, Cephalopods: a, Cardioceras cordiformis 
M. and H.; b, Neumayria henryi M. and H.; c, Belemnites densus M. and H. d-h, 
pelecypods: d, Camptonectes bellistriatus Meek; e, Mytilus whitet Whitf.; f, Gram- 
matodon inornatus M. and H.; g, Pseudomonotis curta (Hall); h, Ostrea strigilecula 
White. 7 and 7, brachiopods: 17, Rhynchonella gnathophora Meek; j,: Lingula 
brevirosira M. and H. 


and sturgeons were numerous, and the initial forms of the bony 
fishes (teleosts), the dominant type, made their appearance. The 
class was distinctly more modern than at the close of the 







Medi, 
NTN 
NNN 


yp 





Fig. 433. A Jurassic coelacanth, Undina gulo, a crossopterygian, about 1/7 
natural size; the outline of the air-bladder is shown just back of the gills and under 
the axis. (Restored by A. Smith Woodward.) 


Paleozoic. Though the fishes doubtless suffered from the reptiles 
which went down to sea in the Trias, it appears that they continued 
in notable abundance and variety. It will be seen later that they 


LIFE 513 


outlived the invading race, and resumed, in large measure, their 
former dominance. 





Fig. 434. A jurassic spookfish or chimeroid, Squaloraja polyspondyla, % 
natural size; from the Lower Lias, Dorsetshire. (Restored by A. Smith Wood- 
ward.) 





Fig. 435. A Jurassic forerunner of the modern Amia, Eugnathus athostomus, 
about 1/7 natural size, from the Lower Lias, Dorsetshire. (A. Smith Woodward.) 





- Fig. 436. Outline and skeleton of Ichthyosaurus quadriscissus. (After Jaekel.) 


(9) Some of the reptiles which had taken to the sea in the pre- 
ceding period had become extinct, while others made their first 
appearance in this period. The ichthyosaurs (fish-like saurians) 
reached their highest development in this period, and seem to have 
swum every sea. Their adaptation to aquatic life is shown in the 
complete transformation of their limbs into paddles (Fig. 436), in 


sT4 THE JURASSIC PERIOD 


the reduction of the outline of the body to fish-like lines and propor- 
tions, in the sharp down-bending of the vertebre at the end of the 
tail for the support of a caudal fin, in the long snout set with teeth 
adapted to seize and hold slipping prey, but not to masticate it, in 
the protection of the eye by bony plates, and, interestingly enough, 
in the development of a viviparous habit that freed them from the 
necessity of returning to land to deposit their eggs. That their food 
consisted in part of invertebrates is evident from the fossil contents 
of the stomachs, the remains of 200 belemnites having been found in 
a single one. There were small as well as large forms of ichthyo- 
saurs, some exceeding 30 feet in length. 

Descended from a different stock, the plesiosaurs (Fig. 437) 
adapted themselves to sea life in another way. The body took on a 









D LOA) SY ¥ tn ( 
| Piet , 
[i f LY 
\ \ A 
OD, 
ie Ls 
SEZE" or 
Fig. 437. Skeleton of Plesiosaurus dolichodeirus Conyb. (Restored by Cony- 
beare.) 


form like that of a turtle, while the neck was elongate, giving rise 
to the epigrammatic description ‘‘the body of a turtle strung on a 
snake.’”? Swimming was chiefly by means of paddles, though some 
forms had a fin-like adaptation of the tail. The elongation of the 
neck was variable, the vertebrz of the neck numbering from 13 to 
76. The neck appears not to have been so flexible as familiar 
illustrations have represented it, nor were the jaws separable and 
extensible as in the case of snakes. This implies either that they 
lived on small prey, or tore their food to pieces before swallowing. 
They were doubtless formidable foes of the smaller sea animals, but 
probably not of the larger. Like ichthyosaurs, they were without 
scales. They ranged from 8 to 40 or more feet in length. 

Marine crocodilians made their appearance late in the period. 
They had undergone a remarkable adaptation to the sea (Fig. 438). 
They were fish-like in appearance, their skins were bare, and their 
tails terminated in a fin like that of the ichthyosaurs. The fore 
limbs were short and paddle-like. The hind limbs were modified 


LIFE 515 





Fig. 438. Restoration of a Jurassic crocodilian, Geosaurus suevicus. (Fraas.) 


but slightly from the land type, perhaps due to the recurring neces- 
sity of visiting the shores for depositing and hatching eggs. Marine 
turtles, so characteristic of the Cretaceous, had not yet appeared. 


Land Life 


Vegetation. The land vegetation of the Jurassic was little more 
than a continuation and expansion of that of the late Triassic, with 
slow progress toward living types. Cycads, conifers, ferns, and 
equiseta were the leading plants, slightly more modernized than 
their Triassic ancestors, but not changed radically... The conifers 
were represented by yews, cypresses, arborvitas, and pines, all of 
which had a somewhat modern aspect, though all the species are 
extinct. 

An interesting feature of the European record is the rather 
frequent occurrence of land plants in marine beds, which implies 
that many trunks, twigs, leaves, and fruits were floated out to sea, 
and that the landward edges of the marine deposits have escaped 
destruction. In the same beds are the remains of many land insects, 
not a few of them being wood-eating beetles. 

Animals. Of the early Jurassic land faunas of North America 
little is known; but in the Morrison beds (perhaps Comanchean, 
p. 504) there is a fauna composed chiefly of dinosaurs. Some of these 
reptiles were large, and some small, and the group as a whole had 
great diversity in many directions. There were not only carniv- 
orous types, which had appeared in the Trias, but numerous herbiv- 


1 Jurassic plants of the United States, with descriptions and illustrations by 
Lester F. Ward; 20th Ann. Rept., U. S. Geol. Surv., pp. 334-430. 


516 THE JURASSIC PERIOD 


orous forms; but among them all there was not a single type which 
was distinctively North American. It is therefore concluded that 
there was freedom of migration between the eastern and western 
continents at this time. 

- Of the carnivores, one of the most common was a type (Fig. 439) 
whose fore limbs seem to have been used chiefly for seizing and hold- 





Fig. 439. A carnivorous dinosaur, Ceratosaurus nasicornis, about 1/40 natural 
size; i.e., length about 17 feet; from the Como beds, Colorado. (Restoration of 
skeleton by Marsh.) 


ing prey, rarely for walking. The animal’s pose was facilitated by 
hollow bones. The head was relatively large, an unusual character 
for a race among which small heads and brains were the fashion. 
Besides the large ones, there were small leaping forms not larger than 
a rabbit. 

The herbivorous dinosaurs, known first in this system, developed 
so rapidly that they soon outranked the carnivorous forms in both 
size and diversity. Most of them were massive, with sub-equal 
limbs and the quadrupedal habit. Some of them (Fig. 440) attained 
a length of 60 feet (possibly more), taking rank among the largest 
of known land animals. These enormous creatures were char- 
acterized by weakness rather than strength, for they were unwieldy, 
their heads and brains small. ‘‘The task of providing food for so 


LIFE S17 


i 


large a body must have been a severe tax on so small a head.’’ The 
largest of all known dinosaurs (Brachiosaurus) had a femur nearly 





WE 
MALES Ss 
Wes 
» Nihese 
és 
Nee 
Ke 
\\ YS. 


— 
SEF Bx i 


Fig. 440. An herbivorous dinosaur, Brontosaurus (A patosaurus): Restoration 
of skeleton by Riggs, nearly 60 feet long; from Wyoming. 


7 feet long. There were other genera of similar nature, and of bulk 
inferior only to these monsters. © 

The typical ornithopod (bird-footed) dinosaurs were bipedal in 
habit, like the carnivores. On the hind limbs there were usually 
only three functional toes, so that they left a bird-like track; the 
fore limbs, however, had five digits. One of the largest of this group 





Fig. 441. Stegosaurus, an armored dinosaur of the Jurassic. Interpreted by 
Charles R. Knight. (Lucas’ Animals of the Past. By permission of the publishers, 
Messrs. McClure, Phillips and Company.) 


518 THE JURASSIC PERIOD 


measured about 30 feet in length, and 18 in height in the walking 
posture. 

The stegosaurs were quadrupedal in habit, and had solid bones. 
Though not so large as some of the preceding, they were curiously 
armored, and formed a very remarkable group that frequented 
England ani Western America. The Stegosaurus of Colorado and 
Wyoming (Morrison beds) was one of the most unique (Fig. 441). 
Its diminutive head and brain imply a sluggish, stupid creature, de- 
pending for protection on its bulk and armor. 

A unique feature of the period was the development of pierosaurs, 
or flying reptiles. Appearing at the close of the Trias in a few yet 
imperfectly known forms, they were at the opening of this period, 





Fig. 442. Rhamphorhynchus phyllurus, a flying saurian. (Restored by Marsh.) 


fully developed flying animals, and later formed a diversified group 
which included long-tailed (Fig. 442) and short-tailed forms (Fig. 
443). With little doubt they sprang from some agile, hollow- 
boned saurian, more or less akin to the slender, leaping dinosaurs. 
Between the ponderous forms (Figs. 440, 441) and the pterosaurs 
(Fig. 442), the Jurassic saurians present strange contrasts. 

Jurassic pterosaurs were small, but their successors ‘attained 
a wing-spread of nearly a score of feet. They were curiously com- 
posite in structure and adaptation. Their bones were hollow, their 
fore limbs modified for flight, their heads bird-like, and ‘their. jaws 
set with teeth, though toothless forms appeared later. They were 
provided with membranes stretched, bat-like, from the fore limbs 
‘to the body and hind limbs, which served as organs of flight (Fig. 
442). The fifth, or as some paleontologists believe, the fourth front 
digit was greatly extended, and supported the wing-membrane. 
The sternum was greatly developed, implying true powers of flight, 


LIFE 510 


a conclusion supported by the occurrence of their remains in marine 
sediments free from other land fossils. Some of them had singular 
elongate rod-like tails, with a rudder-like expansion at the end. 
Pterodactyls (Fig. 443) 
had short tails, and 
were mostly small and 
slender. Fully differ- 
entiated as first found, 
they underwent no 
radical change of struc- 
ture during their career, 
and the steps of their 
remarkable evolution 
are for the most part 
unknown. Flying rep- 
tiles are extremely rare 
among the Jurassic fos- 
sils of North America. 

Turtles, which had 
lived elsewhere since 
the Middle Trias, made 
their first appearance 
in North America in , 
fe torso. beds, and Fig. 443. Skeleton of pterodactyl, Pterodac- 


the crocodilians became tylus spectabilis, from the lithographic stone at 
differentiated into sev- Eichstadt, Bavaria; about 34 natural size. (After 


eral branches. Primi- H- v- Meyer.) 

tive lizards were doubtless abundant, but because of their terrestrial 
habits and small size, they have little representation among the 
fossils, and none have been found in our continent. 

A less bizarre, but really greater evolution, was the differentia- 
tion of true birds. The remote ancestors of the pterosaurs and the 
birds may have been closely allied, but there is no evidence that 
birds descended from pterosaurs. The two are examples of analo- 
gous and parallel evolution, not of relationship. 

The oldest known bird, Archeopteryx macrura (Fig. 444), shows 
clear traces of a reptilian ancestry. From this ancestry it retained a 
long, vertebrated tail, reptile-like claws, teeth set in sockets, biconcave 
vertebree, and separate pelvic bones. On the other hand, its head 
and brain were bird-like, its anterior limbs adapted to flying in bird- 





520 THE JURASSIC PERIOD 


(not pterosaurian) fashion, its posterior limbs modified for bird-like 
walking, and most distinctive of all, it was clothed with feathers. 
The presence of 
feathers, while yet 
the body retained 
so many reptilian 
features, is remark- 
able. But for their 
preservation, it is. 
uncertain whether 
the creature would 
have been classed 
as a bird or reptile. 
\\ The known speci- 
* men was somewhat 
~ smaller than a crow. 
* The marvelous 
AY deployment of 
aquatic and terres- 
trial reptiles and of 
birds makes the 
scanty record of the 
mammals all the 
more singular. Only 
a few jaw bones of 
the size of those of 
mice and rats have 
been found. These 
Ni i low types are re- 

Fig. 444. The earliest ferred, without com- 
known bird, Archeopteryx plete certainty, to 
macrura. The long verte- marsupials. They 


brated tail, the clawed digits . 
of the fore limbs, and the 4PP¢at to have been 


toothed jaws are ancestral insectivorous. 
features to be specially not- The insects of the 
ed. (H. von Meyer.) : 

Jurassic appear to 
have included members of nearly all fossilizable groups not depend- 
ent on flowering plants. 








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Map work. For suggestions as to map work see Laboratory Exercises in 
Structural and Historical Geology, Exercise X. 


CHAPTER XXIV 
THE COMANCHEAN (LOWER CRETACEOUS) PERIOD 


Definition. The history of the Cretaceous period, as formerly 
defined, was complex. At its beginning, the larger part of the 
North American continent was above the sea. During its progress, 
the sequence of events in our continent was somewhat as follows: 
(1) A somewhat widespread warping of the continental surface, 
resulting in extensive submergence in Mexico and Texas, and a lesser 
submergence along the Pacific coast. At about the same time the 
Atlantic and Gulf coasts and some parts of the western interior were 
sites of deposition, though not submerged. Prolonged sedimenta- 
tion followed. (2) Geographic changes which inaugurated a long 
period of erosion that affected the recent deposits as well as older 
formations. (3) Encroachment of the sea submerging the Coastal 
Plain of the Atlantic and the Gulf of Mexico, and presently the 
Great Plains, probably to the Arctic Ocean. On the Pacific coast, 
too, the sea gained on the land. Few greater transgressions of land 
by sea are recorded in the long history of the North American con- 
tinent. A long period of deposition was initiated by the sub- 
mergence. It was succeeded by (4) a widespread withdrawal of 
the waters from the continent, leaving the land area nearly or quite 
as large as now. 

The formations of the Cretaceous period have been divided, 
commonly, into two main series, a Lower and an Upper. To the 
former were referred the deposits of the earlier and lesser submer- 
gence, and to the latter, those of the later and more extensive sub- 
mergence. ‘The distinctness of the Lower and Upper Cretaceous is, 
however, so great that it is more in keeping with the spirit of modern 
classification to regard them as separate systems, and the corre- 
sponding divisions of time as periods. What was formerly called 
the Lower Cretaceous series is here called the Comanchean system. 
The propriety of this classification is the more striking, since it is 
applicable to other continents as well as to our own. 


521 


522 THE COMANCHEAN PERIOD 


Fig. 445. Map showing the distribution of the Comanchean formations 
North America. The conventions are the same as in preceding maps. 





j FORMATIONS AND PHYSICAL HISTORY 523 


FORMATIONS AND PHYSICAL HISTORY 


Atlantic and Gulf border regions. That part of the Coman- 
chean system along the Atlantic coast is called the Potomac series; 
the part along the eastern Gulf coast, where conditions of sedi- 
mentation appear to have been similar, is the Tuscaloosa series. 
Fig. 445 shows that the system outcrops near the inland margin of 
the Coastal Plain. It is the lowest of the Coastal Plain formations. 
Neither the Potomac nor the Tuscaloosa series is believed to repre- 
sent the whole of the period, and the two are not strictly contem- 
poraneous. 

Conditions of origin, and constitution. By the beginning of 
the Comanchean period, both the Appalachian Mountains and the 
area to the east had been degraded well toward base-level, so that 
little warping of the surface appears to have been needed to convert 
_ portions of the coastal lands into sites of deposition, though more 
may have been necessary to provide lands high enough to furnish 
abundant sediments. The peneplanation of the eastern mountains 
during the Jurassic period was no doubt attended by deep decay of 
the underlying rocks, and the consequent accumulation of a heavy 
mantle of residuary earth. The warping which inaugurated the 
Comanchean period seems to have involved a rise of the Appalachian 
tract, and a consequent rejuvenation of the drainage from it, while 
the coastward tract was left relatively low and became a zone of 
lodgment for the sediments brought down by the quickened drain- 
age from the west. Lakes, marshes, etc., probably were features 
of the lodgment area. The deposits consist of gravel (or conglom- 
erate), sand (or sandstone), and clay, largely uncemented. 

The gravel and sand came chiefly from formations to the west. 
Both are arkose (containing particles of crystalline rock, not de- 
cayed when deposited) locally, showing that erosion sometimes 
exceeded rock decay. ‘This suggests high land to the west whence 
the sediments were derived, and is one of the reasons for the belief 
that it was tilted upward at this time. Beds of clay in the Potomac 
series have been utilized extensively, especially in New Jersey,! for 
the manufacture of clay wares. Some of it is notable for its bright 
and variegated colors, black, white, yellow, purple, and red being not 
uncommon. White is to be looked upon as the normal color; the 
others are the result of various impurities, the black being due te 
organic matter. 

1 Cook, Geol. Surv. of New Jersey, 1868, and Kiimmel, 1904. 


524 THE COMANCHEAN PERIOD 


The clay, sand, and gravel are disposed irregularly, doubtless 
the result of the physical conditions where the sedimentation took 
place, conditions which might have existed along the lower courses 
of rivers or at their debouchures, where shore-waters had little 
effect upon them. 

In addition to the clastic sediment, there is a little lignite, and 
some iron ore, and though both are widely distributed, neither 
is of much commercial value. | 

Structure and thickness. The Potomac and Tuscaloosa series 
are nearly horizontal, with a gentle dip seaward. The Potomac 





Fig. 446. Section showing relations of various members of the Coastal series. 
C, Comanchean; K, Cretaceous; L, Eocene; M, Miocene; P/, Pliocene; 0, Quater- 
nary. 


series rests unconformably on Triassic and other formations (Fig. 
446), and the Tuscaloosa on Paleozoic or older strata. Both series 
are overlain unconformably by the Upper Cretaceous. The Poto- 
mac formations reach a thickness of 700 feet in but few places. 
The thickness of the Tuscaloosa series is about twice as great. 

Western Gulf Region. ‘The system is more fully represented in 
Texas than farther east, but its stratigraphic relations are the same. 
The beds appear at the surface over an area distant from the coast, 
dip seaward at a low angle, and are concealed near the coast by 
younger formations. The lower part of the system (the Trinity 
series) is perhaps the time equivalent of the Potomac, while 
the uppermost series (the Wichita) is probably younger than any 
part of the system on the Atlantic coast. Some parts of the system, 
especially the middle (Fredericksburg) are marine, and some ter- 
restrial. The marine part includes much limestone. The system 
here is much thicker than farther east, ranging from 1,000 feet to 
about 4,000. 

From Texas, the Comanchean formations, or some of them, 
originally spread northward into Kansas, northwestward to Colo- 
rado, and westward to Arizona. ‘Though they appear at the surface 
in small areas only, their extent may be considerable beneath younger 
formations. 


FORMATIONS AND PHYSICAL HISTORY 525 


The Comanchean of Mexico is mainly limestone, and, though 
but imperfectly known, has been estimated to have the extraordi- 
nary thickness of 10,000 to 20,000 feet. Its distribution is such as 
to show that a large part of that country was beneath the sea. It 
has been conjectured that the waters of the Atlantic and Pacific 
met over the site of some part of Mexico at this time, but this is 
uncertain. If the oceans were connected, it was probably across 
southern Mexico, or perhaps Central America. At any rate, there 
does not seem to have been free faunal intermigration between the 
Gulf coast and the coast of California. 

Northern interior. The sea is not known to have extended north 
of Kansas during the period; but clastic beds of terrestrial origin, 
and perhaps of Comanchean age, are known at various points farther 
north. The beds in question, the Morrison (p. 504) beds, are best 
known along the Front Range through Colorado and Wyoming, and 
in the Black Hills, though they probably reach northward to Mon- 
tana. If all the beds thought to be the equivalent of the Morrison 
are really so, the formation is widely distributed. These beds are 
regarded by some as Jurassic, and this may be their proper classi- 
fication. 

In Montana, Alberta, and Assiniboia, there are beds (the Koote- 
nay and Cascade formations, etc.) similar in character to those just 
mentioned. ‘They are mainly clastic, and contain some coal. ‘Their 
fossils are mostly of plants of early Cretaceous types. In Mon- 
tana, the Kootenay formation overlies the Morrison. 

To the Morrison and Kootenay formations a lacustrine origin 
has usually been assigned, and there is perhaps no adequate ground 
for questioning this conclusion for some parts of the formations; 
but the character of some of the beds and the nature and distribu- 
tion of their fossils suggest a fluviatile origin for parts, and perhaps 
for large parts, of the series. 

Pacific border. The system (known as the Shastan group) has 
great development in California, where it attains its maximum 
known thickness. It is made up of the Kunoxville series below and 
the Horsetown series above. The deposits are thickest in the 
Sacramento valley. Most of the thick system, including its basal 
beds, bears the marks of shallow-water origin. ‘The Shastan group 
is represented in Oregon also. 

Where the base of the system has been observed, it is uncon- 
formable on Jurassic rocks, or on metamorphic rocks of unknown 


526 THE COMANCHEAN PERIOD 


age. It is overlain unconformably in some places, and without 
apparent unconformity in others, by the (Upper) Cretaceous (Chico 
series). 

Farther north, Lower Cretaceous beds (Queen Charlotte series) 
occur in the Queen Charlotte Islands,' where they have an estimated 
thickness of between 9,ooo and 10,000 feet. In British Columbia, 
the coast line was east of the Coast ranges, and extended farther and 
farther east with increasing latitude, until the ocean swept clean ~ 
across the site of the Cordillera in the early part of the period, and 
extended south along the area which is now the east base of the 
mountains.” The Kootenay formation is perhaps partly contem- 
poraneous with these marine beds. The Comanchean system of 
British Columbia generally rests unconformably on the Triassic 
system, and contains some volcanic material and, locally, coal. 

Farther north, the Lower Cretaceous has not always been sepa- 
rated from the Upper, but the former has extensive development in 
some parts of northern Alaska, where it contains coal. It occurs 
also on the west coast of Greenland, where the beds are thought to 
represent some such horizon as that of the Kootenay, or Potomac. 


Close of the Period 


Considerable changes in the geography of North America brought 
the Comanchean period to a close. Along the Atlantic and Gulf 
borders considerable tracts were converted from areas of depo- 
sition into areas of erosion. The system was somewhat deformed 
and faulted in both Texas and Mexico. In the southern Coast 
Range of California there was folding of the Lower Cretaceous beds, 
accompanied by volcanic activity, while in other places the sea 
spread itself over areas which had been land. Still other areas in 
the west appear to have emerged at this time, and never to have been 
submerged since. 

On the whole, the deformative movements at the close of the 
period were more extensive, so far as present knowledge goes, than — 
those which occurred in the midst of any one of the Paleozoic 
periods as here defined. On stratigraphic grounds, therefore, the 
distinctness of the two systems is clear. The case is hardly less 
clear on the paleontological side. 


1 Dawson, Geo. M., Am. Jour. Sci., Vol. XX XVIII, 1889, pp. 120-127. 
2 Dawson, Science, March 15, 1901; Bull. Geol. Soc. Am., Vol. XII, p. 87. 


FORMATIONS AND PHYSICAL HISTORY 527 


Lower Cretaceous in Other Continents } 


Europe. The deposits in some of the lakes, marshes, estuaries, 
and other lodgment basins which resulted from the geographic 
changes at the close of the Jurassic period in Europe, record the 
transition from that period to the early Cretaceous. ‘The interrup- 
tion of marine sedimentation in Southern Europe was not so general, 
and over considerable areas the Lower Cretaceous succeeds the 
Jurassic conformably, both being marine. 

During the early stages of the Lower Cretaceous, the areas of 
sedimentation were more or less isolated; but later, advances of 
the sea united many of them. The Lower Cretaceous formations 
include all sorts of clastic rocks, together with limestone, glauconitic 
beds, beds of coal (northwestern Germany), and iron ore. They 
embrace, indeed, about all varieties of sedimentary rock except 
chalk, the rock from which the name ‘‘Cretaceous”’ was derived. 
In southern Europe, much of the system is limestone. 

Other continents. In other continents, the Lower and Upper 
Cretaceous have been less clearly differentiated; yet enough is known 
to show that the Lower and Upper Cretaceous systems are, in gen- 
eral, markedly different, both in origin and distribution. Marine 
Lower Cretaceous is well developed in the western part of the 
Andes Mountains. It is widespread also in the northern part of 
South America, but not elsewhere east of the Andes. It is generally 
absent about the borders of the South Atlantic. On the other hand, 
marine Lower Cretaceous beds occur in many places about the 
southern Pacific and Indian Oceans. Lower Cretaceous formations 
of marine origin are widespread also in Siberia and Japan. The 
system is believed to have slight development in the mountains 
of northwestern Africa, where it is really an extension of the Lower 
Cretaceous of southern Europe, and is unconformable beneath the 
Upper Cretaceous, and in South Africa. 

Geographic changes of importance occurred in various parts of 
the earth at the close of the early Cretaceous period, as shown by 
(1) the unconformities between the Lower (Comanchean) and 
Upper Cretaceous systems, as at some points in Europe, north 
Africa, Australia, and South America, and (2) in the differences in 
their distribution. 


1 The term Comanchean is not applied to the Lower Cretaceous formations 
outside of North America, 


528 THE COMANCHEAN PERIOD 


Climate 


In the aggregate, the known fossils of the Lower Cretaceous of 
America are not such as to indicate great diversity of climate. 
Even in Greenland, the climate seems to have been as warm as that 
of warm temperate regions to-day. 

The fresh-water fossils of those deposits of central Europe 
which represent the transition from the Jurassic to the Lower Cre- 
taceous, indicate a climate far from tropical. It would seem to 
have been comparable to that of the temperate portions of America 
to-day. The fossils of lower latitudes denote a warmer climate. 
On the whole, European fossils seem to afford better evidence of the 
existence of climatic zones than those of America. 


LIFE 


Land vegetation. Fossil plants constitute the chief record of 
the life of the early stages of the Comanchean in America. The 
earliest flora was akin to that of the Jurassic, the cycadeans (Fig. 
447), conifers, ferns, and horsetails being the dominant forms. In 
most of Europe, this group held possession of the land throughout 
the period, though angiosperms appeared in Portugal before its 
close. Descendants of Jurassic types of plants also continued 
throughout the period in northwestern America. 

Introduction of angiosperms. This period was marked by one 
of the most radical evolutions in the history of the plant kingdom. 
Angiosperms (p. 685), including both monocotyledons and dicoty- 
ledons, appeared early in the period, and developed so rapidly that 
by the beginning of the next they had overrun the continent. Their 
precise time and place of origin is not known, but present data 
point to the borders of the north Atlantic as the place of origin, 
and the late Jurassic or earliest Comanchean as the time. 

About 400 species of Comanchean angiosperms are known from 
the Atlantic coast. They were in a minority in the lowest Potomac, 
but increase to an overwhelming majority in the upper beds. The 
earliest forms are not really primitive, and throw little light on the 
origin of the group. The majority resemble modern genera, and a 
few (as Sassafras, Ficus, Myrica, and Aralia) are referred to living 
genera. Before the end of the period, figs, magnolias, tulip trees, 
laurels, and other forms referred to modern genera, but not to mod- 
ern species, had appeared. By this time the cycadeans had dropped 


LIFE 529 


to an insignificant place, and the conifers and ferns, while not equally 
reduced, were subordinate to the angiosperms. 

Land animals. The aspect of the vertebrate life was inter- 
mediate between that of the Jurassic and Upper Cretaceous, and, so 


Tage § 1 a PR ES a 
a Roe ia 5 6 == 
SCENTI MET ERS e 


a ahs 





Fig. 447. A cycadean trunk from the Black Hills, Dakota, Cycadeoidea dako- 
tensis Ward, Lower Cretaceous. (Ward.) 


far as it is known, has been sketched already (p. 515). Little is 
known of other forms of terrestrial animal life, but it has been con- 
jectured that the great development of flowering plants was con- 
nected with the existence of abundant insect life. 

Fresh-water fauna. The molluscan fauna of the inland waters 
had assumed a pronouncedly modern aspect, as illustrated in Fig. 


530 THE COMANCHEAN PERIOD 


448. It probably had at- 
tained considerable impor- 
tance through the extension 
of the fresh waters, but the 
record is by no means so 
ample as would be expected 
if the deposits were made 
mainly in lakes and river 


channels. This is an addi- Fig. 448. FRESH-WATER FossIts OF THE 
tional reason for the grow- COMANCHEAN (Lower Cretaceous) from 
8 Montana. a and Jb, Pelecypods: a, Unio 





ing opinion that the terres- farri Stanton; b, Unio douglassi Stanton; 


trial deposits were in con-  c-e, Gastropods: c. Viviparus montanensis 


a Stanton; d, Goniobosis (?) ortmanni Stanton; 
oat res ie Hae e, Campeloma harlowtonensis Stanton. 


transient type, due to overflows, storm-wash, sheet-wash, and other 
forms of more strictly subaérial aggradation. 





Fig. 449. COMANCHEAN FOSSILS OF THE TEXAN PROVINCE. a-c, Echinoids: 
a, Holaster simplex Shum.; b, Diplopodia texanum Roemer; c, Hemiaster dalli Clark. 
d-h, Pelecypods: d, Anatina austinensis Vaughan; e, Homomya austinensis Vaughan; 
f, Trigonia emoryi Conrad; g, Lima wacoensis Roemer; h, Pecten texanus Roemer. 
i-l, Gastropods: 7, Fusus texanus Vaughan; j, Turritella budaensis Vaughan; k, 
Cerithium (?) texanum Vaughan; /, Trochus sp.; m, a coral, Parasmilia texana 
Vaughan. 


Marine faunas. Two very distinct marine faunas are found in 
North America, that of the Mexican Gulf and that of the Pacific 


LIFE | 531 


Coast. The former had its connections eastward with Portugal 
and the Mediterranean region; the latter, northward and westward 
with Asia and Russia, though the boreal element is less conspicuous 
in the upper part (Horsetown). No species common to the two 
provinces is known. The decline of the boreal aspect of the western 





b 

Fig. 450. FossILs FROM THE SHASTAN SERIES (chiefly Knoxville). a-c, Cepha- 
lopods: a, Lytoceras batesii Trask; 6, Phylloceras knoxvillensis Stanton; c, Hoplites 
angulatus Stanton. d-h, Gastropods: d, Astresius liratus Gabb; e, Amberleya dilleri 
Stanton; f, Cerithium paskentaensis Stanton; g, Hypsipleura gregaria Stanton; h, 
Turbo moyonensis Stanton. i-q, Pelecypods: i, Pecten complexicosta Gabb; j, 
Corbula (?) persulcata Stanton; k and /, Aucella piochiit var. orata Gabb; m, A. 
crassicollis Keyserling; n, Astarte californica Stanton; 0, Arca tehamaensis Stanton; 
p, Nucula storrsi Stanton; g, Leda glabra Stanton; r, Rhynchonella whitneyi Gabb, 
a brachiopod. (After Stanton.) 


fauna may have been due to the closing of Bering Strait, thus shut- 
ting off cold currents from the Arctic.’ —The Comanchean faunas 
are said to represent three distinct facies, the reef facies, most con- 
spicuous, the /ittoral, and the deeper water facies. 

1 Stanton, Jour. Geol., Vol. XVIi. 


CHAPTER XXV 
THE CRETACEOUS PERIOD 


FORMATIONS AND PHYSICAL HISTORY 


The Cretaceous period was ushered in, so far as North America 
is concerned, by a notable encroachment of the sea. Cretaceous 
formations are found in (1) the Atlantic Coastal Plain; (2). the 
Coastal Plain of the Gulf; (3) the Great Plains, from the Gulf of 
Mexico to the Arctic Ocean; (4) at many points in the western 
mountains; and (5) over considerable areas along the Pacific coast. 
While its distribution has much in common with that of the Coman- 
chean, it is much more widespread (Fig. 451), and unlike the 
Comanchean, this system is chiefly marine. 


Atlantic border region. Cretaceous formations come to the 


surface in a belt near the western margin of the Atlantic Coastal 
Plain (Fig. 451), just east of the outcrop of the Potomac series. 
The beds have been but little disturbed, and still dip, as when 
deposited, slightly to seaward, and in that direction pass beneath 
younger formations. ‘They are largely of unindurated clay and 
sand, with some greensand marl, which is rather characteristic of 
the system. The distinguishing constituent of this marl is glau- 
conite, primarily a hydrous silicate of potassium and iron,! which 
occurs in grains. Glauconite is now making in some parts of the 
sea, and from the situations in which it is formed, it is inferred that 
the conditions necessary for its development on such a scale as to 
make considerable beds, are the following:? (1) Water of moderate 
depth, too to 200 fathoms being the most favorable; (2) a meager 
supply of land-derived sediment; and (3) the presence of forami- 
nifera. The production of the glauconite seems to be effected by 


1Most Glauconite is impure, and, as it occurs in nature, contains several 
other ingredients. 


2 For brief summary concerning the origin of greensand marl, see Clark, Jour. 
Geol., Vol. II, p. 161. For a fuller account, see Challenger Report on Deep Sea 
Deposits. 


532 


Pim 


FORMATIONS AND PHYSICAL HISTORY 533 





Fig. 451. Map showing the distribution of the Cretaceous formation in North 
America. The conventions are the same as in preceding maps. 


534 THE CRETACEOUS PERIOD 


chemical changes in sediments perhaps as the result of decomposi- 
tion of the organic matter contained in the foraminiferal shells. 

The subdivisions now generally recognized are the following, 
commencing with the lowest: 1. Matawan formation; 2. Monmouth 
formation; 3. Rancocas formation; 4. Manasquan formation. ‘These 
formations are not all continuous throughout the coastal region, 
and all the formations show notable variations when traced along 
their strikes. Their aggregate thickness nowhere exceeds a few 
hundred feet. 


Eastern Gulf border. The outcrop of the Cretaceous formations ~ 


of the eastern Gulf states is shown in Fig. 452. Near the Missis- 


Kj 


: 


WAY 
Vda, 


= N 
=N 
=X 


LLG, 





Fig. 452. Map showing the positions of the several members of the Comanchean 
and Cretaceous systems in Alabama and adjacent states. C, Tuscaloosa series 
(Comanchean); Ke, Eutaw formation; Ks, Selma chalk; Kr, Ripley formation; 
Tr, Tertiary. (After Smith.) 


sippl, the belt of outcrops extends northward to Kentucky. Meager 
remnants (outliers) are found even north of the Ohio, in southern 
Illinois. 

In Alabama, where the Gulf Coast part of the system is best 


known, there are three principal divisions: the Eutaw below 


il 


FORMATIONS AND PHYSICAL HISTORY 535 


(mainly clays and sands, some greensand, 300 feet), the Selma 
Chalk (1,000 feet) in the middle, and the Ripley (mainly sand, 200~500 
feet) above. The Eutaw is believed to be the equivalent of the 
Matawan formation of the Atlantic coast, and the Ripley is thought 
to be older than the Rancocas. The Cretaceous beds of the Gulf 
coast have been disturbed more than the corresponding beds along 
the Atlantic coast. They have been bent into low anticlines and 
synclines in some places (Alabama), and faulted to a slight extent. 

Western Gulf region. The general stratigraphic relations of the 
system here are the same as farther east, but deposition seems to 
have been well under way in Texas before the oldest exposed beds 
of the system farther east were laid down. The system has a 
maximum thickness of about 4,000 feet. Three principal subdivis- 
ions are recognized: (1) The Dakota; (2) the Colorado; and (3) the 
Montana. ‘The Dakota formation, 600 feet and less thick, is largely 
of sandstone, with some lignite, and is, for the most part, of non- 
marine origin. The Colorado series contains much limestone (or 
chalk) of marine origin. Its thickness is about 1,000 feet. The 
Montana series is more largely clastic, and from it the oil of the 
Corsicana oil field of Texas is derived. Locally, the system is much 
faulted. From Texas it is continued northward into Arkansas, and 
westward into New Mexico. 

The Cretaceous of the western Gulf region differs from the cor- 
responding system farther east in its greater thickness, and in its 
greater proportion of calcareous matter, largely in the condition of 
chalk. Of limestone or chalk, the Cretaceous of the Atlantic coast 
contains little, that of the eastern Gulf region (Alabama and Mis- 
sissippi), more, and that of Texas much; nor is the chalk confined 
to the Gulf region, as will be seen. 

Western interior. One of the standard sections of the Creta- 
ceous system of the western interior consists of the following sub- 
divisions, commencing at the bottom: 1. Dakota; 2. Colorado 
(including the Benton and the Niobrara formations); 3. Mon- 
tana (including Ft. Pierre and Fox Hills); and 4. Laramie. This 
classification, however, does not fit all parts of the west. 

The Dakota formation, mainly of non-marine origin, is wide- 
spread in the Great Plains, though most of it is buried. It extends 
westward beyond the Rocky Mountains at many points. The 
formation is largely sandstone, though it contains much conglom- 
erate and clay, and some lignite. It is perhaps to be regarded as 


536 THE CRETACEOUS PERIOD 


the joint product of subaérial and fluviatile deposition. The pres- 
ence of bird tracks in Kansas, and the widespread abundance of 
fossil leaves of angiosperms, in a condition which precludes much 
transportation, imply subaérial sedimentation to a notable extent 
at least. The upper part of the formation carries some marine 
fossils. North of Texas the formation is in apparent conformity 
with the Comanchean in some places, though in others it rests on 
older formations. 

The Dakota sandstone is an important source of water in the 
semi-arid plains. The water enters where the sandstone outcrops. 
near the mountains, and follows the beds down their dip to the east- 
ward. Along the east base of the Rocky Mountains, where the beds 





Fig. 453. A group of concretions weathered out from the Dakota sandstone. 
Near Minneapolis, Kan. (Schaffner.) 


have been tilted, the less resistant formations associated with this 
sandstone have been removed or worn down, leaving the outcropping 
edges of this formation as ridges or “‘hogbacks” (Fig. 93), char- 
acteristic of the east base of the Rocky Mountains much of the way 
from New Mexico to Canada. . 

The Colorado series records an extensive invasion of the western 


FORMATIONS AND PHYSICAL HISTORY 537 


interior by the sea, the invasion going so far, probably, as to estab- 
lish connection between the Gulf of Mexico and the Arctic Ocean, 
over the site of the Great Plains. Clastic formations predominate 
in the Colorado series as a whole, but there are beds of chalk com- 
parable to those of Europe, from Texas to South Dakota. The 
aggregate thickness of the series is locally as much as 3,000 feet, as 
strata are measured, though its average thickness is much less. 

The origin of chalk. There has been much difference of opinion 
concerning the origin of chalk. Its resemblance to the foraminif- 
eral ooze of the deep seas long since led to the belief that it was a 
deep-sea deposit; but closer examination has thrown doubt on this 
conclusion, for the differences between the chalk and foraminiferal 
ooze are as striking as their likenesses. Both consist largely of 
the shells of foraminifera, but with them are associated shells of 
other types. The echinoderms, the sponge spicules, and the secre- 
tions of certain microscopic plants of the chalk correspond in a 
general way with those of the oozes now forming, and are consistent 
with the deep-water origin of the chalk. The molluscan shells of 
the chalk, on the other hand, seem to point with clearness to water 
no more than 30 to 50 fathoms deep. The distribution of the chalk 
and its relations to other sedimentary beds indicate its deposition 
in shallow water, not in water comparable in depth to that in which 
oozes are now formed. On the whole, the balance of evidence is in 
favor of the view that the Cretaceous chalk was deposited in rela- 
tively shallow water. The conditions for its origin seem to have 
been clear seas, with a genial climate. Its materials may accumu- 
late as well on the bottom of a shallow sea as on the bottom of a 
deep one, if clastic sediments are absent.’ 

Following the Colorado epoch there were changes in the sedi- 
mentation and in the life of the western interior sea. The Mon- 
tana series is chiefly clastic, but the area of sedimentation was 
somewhat contracted. The beds are largely marine, and the water 
shallowed as the epoch progressed. Land formations also are found 
in the series. Local beds of coal give evidence of marshy conditions. 
Like other parts of the system, the Montana series abounds in con- 
cretions, some of which attain great size. The thickness of the series 
is variable, and its maximum is great. From 8,700 feet in Colorado, 
it thins to 200 feet in some parts of the Black Hills. 

Deposition continued in the Great Plains and to some extent 

1¥For fuller statement of this subiect see Earth History, Vol. III, p. r4o. 


538 THE CRETACEOUS PERIOD 


west of them through the last epoch of the Cretaceous period, but 
most of the sedimentation was non-marine. Fresh-and-brackish- 
water beds are widely distributed. 

The Laramie series records the transition from the marine con- 
ditions of the Montana epoch to the fresh-water and land condi- 
tions of the Tertiary in the same region. This change did not take 
place everywhere at the same time. ‘The series consists primarily 
of sandstone and shale, with some conglomerate; but with these 
clastic formations there is much coal. Both shale and coal are © 
more abundant below than above, while in the upper part of the ~ 
series conglomerate is not rare. The thickness of the Laramie 
series is estimated at 1,000 to 5,000 feet, exclusive of the transi- 
tion (Mesozoic-Cenozoic) beds to be mentioned below. In not 
a few places there is an unconformity in the great group of strata 
formerly classed as Laramie, and there is difference of opinion as to 
whether the part above this unconformity should be called Lara- 
mie. The present tendency is to regard it as Eocene.! 

In a considerable area of northeastern Wyoming, and in a large 
area farther north, some of the Laramie lignite has been burned in 
the ground. The burning was relatively recent, and locally is still 
in progress. ‘The firing appears to have taken place at the outcrops 
on hill and valley slopes. The burning was accompanied by fusion, 
semi-fusion, and baking, resulting in lava-like slag and brick-red 
banks of indurated clay. 

Coal. The Cretaceous is pre-eminently the coal period of the 
west. Coal-beds occur in every one of its principal divisions in 
this part of the continent. The total amount of coal, chiefly in the 
Laramie, is perhaps comparable to that in the Pennsylvanian sys- 
tem, though the coal is not now so accessible, and its quality not 
so good. It is estimated that along the east and west bases of the 
Rocky Mountains there are more than 100,000 square miles of coal- 
bearing lands, and Colorado alone is estimated to have 34,000,000- 
ooo tons of available coal,? most of which is Cretaceous. The coal 
is largely lignite, though in Colorado not a little of it has been 
advanced to coking bituminous coal, and even to anthracite, where | 


1The Laramie question is well reviewed by Cross, Washington Acad. of Sci., 
Vol. XI, pp. 27-45, 1909. Other recent discussions by Veatch are found in Am. 


Jour. Sci., Vol. XXIV, (1907), p. 18, and Jour. Geol., Vol. XV, 1907. See footnote — 


Pp. 539- 
2 Storrs, 22d Ann. Rept., U. S. Geol. Surv., Pt. ITI. 


FORMATIONS AND PHYSICAL HISTORY 539 


it has been affected by intrusions of igneous rock. The areas of 
Laramie coal are indicated in Fig. 438. 

Transition beds between Mesozoic and Cenozoic.! There are 
divers, more or less local, terrestrial formations in the west which 
have been referred now to the Cretaceous (Laramie,— or more 
exactly, to the upper Laramie or post-Laramie), now to the Tertiary 





_ Fig. 454. An outcropping ledge of clay, hardened by the burning of the coal- 
bed below. Except in the immediate vicinity of the burnt-out coal-bed, the clay 
is not indurated. Near Buffalo, Wyo. (Blackwelder.) 


(Eocene). These formations are, generally speaking, unconform- 
able on the Laramie, and in some places seem hardly separable from 
the recognized Tertiary? (Fort Union). Their reference to the 
Eocene seems to be justified both on stratigraphic and paleontologic 
grounds, so far as present data are concerned. 

Pacific coast. The Cretaceous system is represented on the 

1 The questions involved in the formations here referred to are discussed in 
the following recent papers: Stanton, Am. Jour. Sci., Vol. XXX, and Wash. Acad. 
Sci. Vol. XI; Knowlton, Wash. Acad. Sci., Vol. XI,and Am. Jour. Sci., Vol. XXXV; 
Stone and Calvert, Econom. Geol., Vol. V; Lee Am. Jour. Sci., Vol. XX XV; and 
Cross, Wash. Acad. Sci., Vol. XI. 

2 Here belong the Arapahoe, Denver, Raton, Monument Creek and perhaps 
other beds of Colorado, the Carbon, Evanston, and Lance (Ceratops) beds of 
Wyoming, and the Lance formation, and part of the Livingston beds of Montana. 


540 THE CRETACEOUS PERIOD 


Pacific coast by the marine Chico series. At the time of its origin, 
this series probably extended along the coast from Lower California 
to the Yukon. ‘The Chico series rests on the Shastan unconformably 
in some places, and overlaps it at others. The fauna of the Chico 
series is littoral. Its oldest portion is older than the fauna of the 
Colorado series, and its youngest is older than the fauna of the 
youngest Cretaceous beds. 


Close of the Period 


About the close of the Cretaceous period a series of disturbances. 
was inaugurated on a scale which had not been equaled since the 
close of the Paleozoic era. These changes furnish the basis for the 
classification which makes the close of this period the close of an era. 
These disturbances continued into later times, but the close of the 
Cretaceous may be said to have been the time when the changes 
had advanced so far as to make themselves felt profoundly. They 
consisted of deformative movements, a part of which were orogenic, 
and of igneous eruptions on an almost unprecedented scale. 

General movements. In the closing stages of the period, the 
sea which had lapped over the Coastal Plains of the Atlantic and 





Fig. 455. Section showing the position of the Cretaceous beds in western 
Oregon. Mg, meta-gabbro ot unknown age; sp, serpentine; as, amphibolite schist; 
Jr, Jurassic (?); Km and Kmw, Cretaceous; Eu, Eocene; Hd, Eocene diabase. 
(Diller, Roseburg, Ore., folio, U. S. Geol. Surv.) : 





the Gulf of Mexico was withdrawn toward the abysmal basin. At 
about the same time, the Appalachian Mountains, which had been 
reduced to a peneplain by this time, were bowed up again. 

It is probable that most of the Cordilleran region was elevated 


FORMATIONS AND PHYSICAL HISTORY 541 


bodily at this time, though not to a great height. Without further 
details, it may be said that enough is known to make it probable 
that a large part of the continent was affected by deformative 
movements of a gentle sort. 

Orogenic movements. The growth of mountains by folding 
probably was in progress in the closing stages of the Cretaceous 
period from Alaska on the north to Cape Horn on the south,— more 
than a quarter of the circumference of the earth. At the same time 
folding movements probably affected the Antillean mountain sys- 
tem,' between the southern end of the Cordilleran and the northern 
end of the Andean systems, for in several of the Antillean islands 
later formations rest unconformably on the deformed Cretaceous 
beds. Where the Eocene rests conformably on the Laramie,. the 
disturbances of this time are not clearly distinguishable from those 
of later date, which increased the folding initiated in this epoch. 
Some of the folded ranges of the western mountains began their 
history at this time, others had a new period of growth, and still 
others date from a later time; yet the close of the Laramie was a 
time of general orogenic movement in the western part of North 
‘America. The Rocky Mountain system may be said to have had 
‘its birth at this time. That these mountains are not older is shown 
by the deformation of the Laramie beds along with those of greater 
age. That some of the folding was not younger is shown by the 
lesser deformation of the Tertiary beds in the same region. 

Faulting. The growth of mountains at the close of the Creta- 
ceous was accompanied by faulting on a somewhat extensive scale 
throughout the region of movement, though the faulting of this time 





Fig. 457. Section in northern Montana, showing Proterozoic rock, A, thrust 
over Cretaceous, K. Subsequent erosion has removed much of the overthrust 
beds, but Chief Mountain is a remnant of them. 


cannot be distinguished everywhere from that of later date. In 

the Rocky Mountains of British Columbia, one overthrust fault has 

been located which crowded the Cambrian rocks obliquely up over 

the Cretaceous. The horizontal displacement is estimated to be 
1 Hill, Nat. Geog. Mag., Vol. VII, p. 175. 


542 THE CRETACEOUS PERIOD 


as much as seven miles,' and the throw 15,000 feet. Near the na- 
tional boundary, the displacement along what appears to be the same 
fault crowded the Proterozoic up over the Cretaceous ? by a move- 
ment of equal magnitude (Fig. 457). The exact date of these faults 
has not been determined, but was, perhaps, mid-Tertiary. 





Fig. 458. Chief Mountain. (Willis, U. S. Geol. Surv.) 


Igneous eruptions. The close of the Cretaceous was marked by 
the inauguration of a period of exceptional igneous activity, con- 
tinuing far into the Tertiary. During this period, great bodies of 
igneous rock, both extrusive and intrusive, were forced up. Erup- 
tions occurred in other lands at about the same time. 


Upper Cretaceous of Other Continents * 


Europe. The distribution of the Upper Cretaceous strata of 
Europe shows that extensive transgressions of the sea occurred at 
the beginning of the period, for in some parts of the continent 
marine Cretaceous formations overlap all older Mesozoic systems. 
During the closing stages of the Upper Cretaceous, fresh-water beds 


1 McConnell, Geol. Surv. of Canada, Vol. II, Rept. D, p. 33, 1886. 

? Willis, Bull. Geol. Soc. of Am., Vol. XIII, pp. 307, 331-335. 

3’The term Comanchean has not been applied outside of North America, and 
the Cretaceous system will therefore be referred to as Upper Cretaceous. 


FORMATIONS AND PHYSICAL HISTORY 543 




















































































































































































































Fig. 459. Map and section showing relations of igneous rock to the Cretaceous 
formations in the Crazy Mountains of Montana. ‘The section is along the line AB 
of the map. Klv, Livingston formation; d2, diorite; gr, granite. The especial 
feature of the map is the extraordinary number of dikes radiating from the central 
intrusion, di. Length of section about 20 miles. (Livingston and Little Belt, 
Mont., folios, U. S. Geol. Surv.) 


appear in localities (Alpine region) where marine sedimentation had 
been in progress earlier in the period, showing that the movements 
which were to mark the close of the era were making themselves 
felt. Limestone is the dominant sort of rock in the Upper Creta- 


544 THE CRETACEOUS PERIOD 


ceous of southern Europe, showing that clear seas still prevailed, as in 
the Early Cretaceous period. From a characteristic genus of fossils, 
much of the limestone is known as Hippurite limestone. In the 
system farther north, there is more clastic material. 

The most notable petrographic feature of the Upper Cretaceous 
of Europe is the chalk. Both in England and France it attains an 
aggregate thickness of several hundred feet, though much of it is 
far from pure. It grades into marls and clays on the one hand, and 
into sandstone on the other. Chalk is, however, by no means 
co-extensive with the system, for it has little development outside 
of the Anglo-French area. Greensand occurs in the Upper Creta- 
ceous as well as in the Lower. 

Asia. The submergence of Europe and North America at the 
beginning of the Upper Cretaceous finds its parallel in other conti- 
nents. There are extensive areas of Hippurite limestone in south- 
western Asia, closely connected with that of Europe on the one hand, 
and with that of North Africa on the other. The Himalayan region 
seems to have been still beneath the sea, for Upper Cretaceous forma- 
tions are found in the mountains at great elevations. South of 
these mountains there appears to have been a large tract of land, in- 
cluding much of the peninsula of India, which has been thought 
to have stretched southwest to Africa; but the configuration of 
the sea-bottom does not lend this view much support. 

Upper Cretaceous beds occur on the coast of China, and in 
Japan. In many places they rest on formations older than the 
Lower Cretaceous, and therefore record an increased submergence 
dating from the beginning or early part of the Upper Cretaceous. 
On the other hand, northern Asia, which was largely submerged 
during the earlier Cretaceous period, was largely land during the 
later. Late in the Upper Cretaceous occurred the extensive lava- 
flows of the Deccan. These flows, 4,000 to 6,000 feet in thickness, 
cover an area of something like 200,000 square miles, and are the 
most stupendous outflows of lava recorded. The fossils in 
sediments interbedded with the lava show that the flows were 
subaérial. | 

Africa. In northern Africa, the Upper Cretaceous beds overlie 
the Lower unconformably, and spread southward, covering most of 
the desert, and so indicating great submergence in the north African 
region. South of the Sahara, no Upper Cretaceous beds are known 
except in a few small areas about the coast, where they rest on 


- 


FORMATIONS AND PHYSICAL HISTORY 545 


crystalline schists with no Lower Cretaceous beds beneath, so far as 
now known. 

South America. In South America the sea invaded eastern 
Brazil, where marine Upper Cretaceous beds cover and overlap the 
non-marine Lower Cretaceous. In some parts of Brazil, however, 
the Upper Cretaceous is represented by fresh-water beds only. 
Farther west, marine Upper Cretaceous beds rest unconformably 
on the Lower Cretaceous, and form the summits of parts of the 
eastern Andes, occurring up to altitudes of 14,000 feet at many 
points, and locally even higher. There appears to have been great 
volcanic activity in the Andean system (Chile and Peru) during the 
late Cretaceous. 

Australia. The phenomena of Australia are in harmony with 
those of other continents. The Upper Cretaceous beds are wide- 
spread, locally resting on formations older than the Lower Creta- 
ceous. Furthermore, the Upper Cretaceous (Desert sandstone) is 
in many places unconformable on the deformed Lower Cretaceous. 

General. In general it may be said that there was little marine 
sedimentation in the Late Cretaceous period north of the parallel 
60° north, where the Jurassic and Lower Cretaceous systems are 
more widespread. Between the parallels of 20° and 60°, on the 
other hand, the zone where marine Lower Cretaceous is but slightly 
developed, the Upper Cretaceous system is widespread. Outside 
of China, the Upper Cretaceous system is wanting over no consider- 
able land-area within these limits. In the equatorial and south 
temperate zones, the Upper Cretaceous seas were also expanded 
much beyond the limits of the waters of the preceding period. 


Climate 


The climate of North America throughout most of the Creta- 
ceous period seems to have been rather uniform and warm through 
a great range of latitude. In Greenland, Alaska, and Spitz- 
bergen, climatic conditions seem to have been similar to those 
in Virginia. Toward the close of the period the temperature was 
perhaps lower, for the Laramie flora is a temperate, rather than a 
tropical, one. The fresh-water fossils of central Europe indicate a 
climate comparable to that of Malaysia. As this seems to have 
been a period of low land, widely extended epicontinental seas, 
extensive calcareous deposits, and slow consumption of carbon 


546 THE CRETACEOUS PERIOD 


dioxide in the carbonation of rock, there was a combination of 
conditions regarded as favorable for a mild and uniform climate. 


LIFE 

Land plants. Angiosperms predominated in North America at 
the beginning of the Cretaceous, and during the period genera now 
living came to be numerous, giving the flora a modern aspect. 
Among the living genera which made their appearance were those 
which include the birch, beech, oak, walnut, sycamore, tulip-tree, 
and maple. Among the gymnosperms there was a notable devel- 
opment of the sequoias, which now include the giant trees of Cali- 
fornia. Of special interest was the presence of genera in Europe and 
the United States which are now confined to the southern hemi- 
sphere. 

Toward the close of this period, monocotyledons first became 
abundant, so far as the record shows. Palms were plentiful, even 
in northerly latitudes, before the close of the period. Of greater 
importance because of their relations to the evolution of grazing 
animals, was the appearance of grasses, which attained prominence 
later. | 

It is worthy of remark that the introduction of dicotyledons, 
the great bearers of fruits and nuts, and of monocotyledons, the 
greatest grain and fodder producers, was the groundwork for a 
profound evolution of land animals. A zodlogical revolution, as 
extraordinary as the botanical one, might naturally be anticipated; 
but it did not follow immediately, so far as the record shows. ‘The 
reptiles seem to have roamed through the new forests as they had 
through the old, without radical modification. But with the open- 
ing of the next era, the anticipated revolution in the animal life of 
the land made its appearance, and advanced with great rapidity. 

The new flora spread widely. The European flora was very 
much like the American, and there was a close resemblance between 
the plants of mid-Greenland (70°-72° Lat.) and those of Virginia, 
indicating climatic conditions of remarkable uniformity. Not only 
this, but the flora was of a sub-tropical type. 

Land animals. ‘The terrestrial animals had the same general 
aspect as in the preceding period. Dinosaurs still retained the lead- 
ing place among land reptiles, though carnivorous forms were less 
abundant and varied than before. Among them was a leaping, 
kai garoo-like form with a length of 15 feet. The most singular 





Fig. 460. GROUP OF FOSSIL LEAVES OF TYPICAL CRETACEOUS PLANTS FROM 
THE DAKOTA HORIZON. @, Liriodendron giganteum Lesq.; b, Nyrica longa Heer; c, 
Magnolia pseudoacuminata Lesq.; d, Sterculia mucronata Lesq.; e, Quercus suspecta 
Lesq.; f, Viburnum inequitaterale Lesq.; g, Betulites westi, var. subintegrifolius Lesq.; 
h, Sassafras subintegrifolium Lesq.; i, Ficus inequalis Lesq. 


dinosaurian development was in the herbivorous branch. Some of 
the forms were very large, of quadrupedal habit, with enormous 


548 THE CRETACEOUS PERIOD 





Fig. 461. Triceratops prorsus Marsh, from the Laramie Cretaceous. (Froma 
painting by C. R. Knight in the U. S. National Museum.) 





Fig. 462. Spoonbill Dinosaurs of the Cretaceous Hadrosaurus mirabilis (Leidy) 
as interpreted by Knight. (Osborn, Copyrighted by the Am. Mus. of Nat. Hist.) 


LIFE 549 


skulls which extended backwards over the neck and shoulders in a 
cape-like flange (Fig. 461). Added to this was a sharp, parrot-like 
beak, a stout horn on the nose, and a pair of large pointed horns on 
the top of the head. One of the larger skulls measures eight feet 
from the snout to edge of the cape. This excessive provision for 
defense was accompanied by a very small brain cavity. Marsh 
remarks that they had the largest heads and the smallest brains of 
the reptile race. They were doubtless stupid and sluggish. . The 
ornithopod division was well represented (Fig. 463). .Their-hinder 
parts were large, their limbs were hollow, and their footprints indi- 
- cate that eaey walked in kangaroo-like attitude. ; 


a) 
SAY 











Ker wk 
OY 277i 
BK SY Aes 
SAS YY A 
a SX D ee on 
Qh une At 
Sa Ed 
- 


“Fig. 463. A Cretaceous dinosaur of the ornithopod division, Claosaurus 
annectens. (Restored by Marsh.) 


Terrestrial turtle remains are found in the Dakota sandstone, and 
the fossils of species inhabiting fresh waters in the late Cretaceous 
deposits of Canada. Of true lizards, only one late Mesozoic form 
is known, and that of small size and uncertain affinities, from the 
Laramie. Snakes made their first appearance, so far as known, in 
the later part of the period, but they were small... Crocodiles under- 


550 THE CRETACEOUS PERIOD 


went a marked change early in the period, developing into the modern 
forms, though some of the old types lived on. 

Flying reptiles made so distinct an advance in specialization 
that Williston regards them as having come to excel all other flying 





Fig. 464. A Cretaceous pterodactyl, Nyctosaurus gracilis Marsh, about 1/10 
natural size, Niobrara Cretaceous, Kansas. (Restored by Williston.) 


vertebrate animals. Some had a wing-spread of perhaps 20 feet. 
In some of the genera (Fig. 464) the development of the front parts 
was great, while the hind parts were so very small and weak that it 
is doubtful whether they could stand on their feet alone. The 
Cretaceous forms were all short-tailed, and for the most part tooth- 
less. Their bills resembled those of modern :birds. 

Terrestrial birds existed, but their record is meager. There 
were some curious aquatic forms, which will be mentioned with 
the sea life. The mammals thus far recovered from the Cretaceous 





Fig. 465. A Cretaceous mosasaur, Platacarpus corypheus Cope, restored by 
Williston; from Upper Cretaceous, Kansas. 


indicate little advance on those of the Jurassic, and they appear to 
have played very little part in the fauna of the period. | 
Sea life. Vertebrates. The ichthyosaurs and plesiosaurs which 


LIFE 551 


had dominated the Jurassic sea lived on into this period. The 
former became insignificant soon after its beginning; but the plesio- 
saurs attained their highest development and perhaps their greatest 
size at this time. The American plesiosaurs indicate lack of inter- 
migration between this continent and Europe. 

The aquatic branch of the scaled saurians (Squamata) became 
veritable sea serpents. The long-necked, lizard-like reptiles of the 
Comanchean period were the forerunners, and perhaps the direct 
ancestors, of a family (the mosasauritans, Fig. 465) which flourished 
in the Cretaceous, and ranged from the Americas to Europe and 
New Zealand. Their short career seems to have ended with the 
period, and no direct descendants are known. 

Marine turtles seem to have appeared first in this period, and to 
have had many forms. Some of them had skulls larger than those 
of horses, and their shells must have been fully twelve feet across. 








Fig. 466. Champsosaurus, from the Laramie of Montana. Length, about six 
feet. (After Brown.) 


In the long interval between the first known appearance of birds 
in the Jurassic, and the later Cretaceous when they reappeared, 
important changes took place, among which was the loss of the 
elongate, bilaterally feathered tail. The Jurassic birds were terres- 
trial, while the Cretaceous were aquatic. The Cretaceous birds 
include about 30 species belonging to two widely divergent orders, 
Hesperornis and Ichthyornis. The former (Fig. 467) were large, 
flightless divers, with aborted wings and remarkable legs. The legs 
were not only very powerful, but the bones of the feet were so joined 
to them as to allow the feet to turn edgewise in the water when 
brought forward, thus increasing their efficiency as paddles. Fur- 
thermore, the legs were so joined to the body frame as to stand out 
nearly at right angles to it, like a pair of oars, instead of being under 
the body like walking legs. Apparently, walking as well as flying 


552 THE CRETACEOUS PERIOD 





Fig. 467. Restoration of the great toothed diver of the Cretaceous, Hesperornis, 
by Gleeson. (From Lucas’ Animals of the Past; by permission of the publishers, 


McClure, Phillips and Co.) 


had been abandoned, and the bird was adapted to swimming and 
diving only. The jaws had teeth set in grooves like those of primi- 


tive saurians, and in other respects 
were like the jaws of snakes. As some 
of these strange birds attained a length 
of six feet, they were doubtless formid- 
able enemies to the sea life on which 
they chose to feed. They have been 
found in Kansas, Montana, North 
Dakota, New Jersey, and England, 
and probably frequented epicontinental 
seas somewhat widely. 

The second type Ichthyornis (Fig. 
468) was scarcely larger than a pigeon, 
and had great power of flight, as in- 
dicated by the strong development of 
the wings and keel. At the same time, 
their legs and feet were small and 
slender. They had teeth in sockets. 





Fig. 468. Ichthyornis victor, 
a Cretaceous toothed bird of 


flight, 1/10 natural size. 
stored by Marsh.) 


(Re- 





For explanation of Figure, see top of page 554. 


554 THE CRETACEOUS PERIOD 


Fig. 469. CRETACEOUS Fosstts. a-e, Echinoderms: a, Pedinopsis pondi 
Clark; b, Cassidulus subquadratus Con.; c, Botriopygus alabamensis Clark; d and e, 
Salenia tumidula Clark. f, g, and h, Pelecypods: f, Ostrea soleniscus Meek; g, 
Idonearca nebrascensis Owen, allied to the arcas of to-day; h, Inoceramus vanuxemi 
M. and H. 7i-l, Gastropods: 7, Neptunella intertextus (M. and H.); 7, A phorrhais 
ee (White); &, Drepanochilus nebrascensis (E. and S.); 1, Pyropsis bairdi (M. 
and H. 





Fig. 470. CRETACEOUS CEPHALOPODS: 4a, Nautilus meehanus Whitf., one of the 
simplest types of closely coiled cephalopods; b, Helicoceras stephensoni Whitf., an 
ammonite coiled in a heliciform spiral, and c, its highly complicated suture; d, 
Prionotropis woolgari (Mantell), a normal ammonite with ornamented shell, and e, 
complex sutures; f, Ptychoceras crassum Whitf., an ammonite shell which is recurved 
upon itself, but not coiled; g, suture of f; k, Scaphites nodosus Owen, an ammonite 
showing as light tendency to uncoil in the last volution; z, Baculites grandis M. and H. 


Their biconcave vertebre and other skeletal features, as well as 
their small brains, suggest reptilian relationships. Their habitat 
was the same as that of Hesperornis, and yet the two were farther 
apart, structurally, than any two types of birds now living (Marsh). 


LIFE oR 


The old types of fishes gave place to new ones (the teleosts) dur- 
ing this period. This change set in during the Comanchean, and 
was complete by the middle of the Cretaceous, though representa- 
tives of the older types lived on. 

Invertebrates. The most notable departure from the preceding 
ages is the prominence of foraminifers among the fossils. They 
made large contributions to the chalk of the period, and they were 
concerned in the formation of the greensand, scarcely less char- 
acteristic of the period than the chalk. While some of these mi- 
nute organisms live on shallow bottoms, on fixed alge, and in 
abysmal water, they are chiefly inhabitants of the surface waters of 
the open sea. 

Sea-urchins (a-e, Fig. 469) were quite abundant, and lent one 
of its characteristic aspects to the fauna. Corals and crinoids, so 
long associated with clear seas, were not plentiful. In the clastic 

formations, pelecypods (f—-h) and gastropods (i-1) abound (Fig. 469). 
It will be seen by a glance at Fig. 469 that they have a modern 
appearance. Cephalopods were still abundant, though ammonites 
were in their decline and showed erratic forms, attended by excessive 
ornamentation, comparable to that which marked corresponding 
stages of the trilobites and crinoids. Odd forms of partial uncoiling, 
or of spiral and other unusual forms of coiling, were common (Fig. 
470). Interesting forms, perhaps to be classed here, were the 
Baculites (i), which resumed the straight form of the primitive 
Orthoceras, while retaining the very complicated sutures of the 
Ammonites (c). 

Map work. Folios of the U. S. Geol. Surv. and Laboratory Exercises in 
Structural and Historical Geology, Exercise XI. In the folios of the U. S. Geol. 


Surv. both Comanchean and Cretaceous are classed as Cretaceous, though the two 
are distinguished, in some cases, in the text and on the maps. 


THE CENOZOIC ERA 


CHAPTER XXVI 
THE EOCENE AND OLIGOCENE PERIODS 


The remaining periods of geological history constitute the 
Cenozoic era, or the era of modern life. The earlier part of the era 
is called the Tertiary, and the later the Quaternary. The Tertiary 
is variously subdivided, as shown below: 


Recent or Human. Post-glacial formations 
Quaternary Pleistocene or Glacial. Glacial formations 
and non-glacial deposits of glacial age 


Cenozoic I II Ill 
Era Pliocene Pliocene N 
: Miocene Miocene pete 
Tertiary ' 
Oligocene Eo E 
ety cene ocene 


FORMATIONS AND PHYSICAL HISTORY 


There is much to be said for a two-fold division of the Tertiary, 
the first including the Eocene, Oligocene, and Early Miocene, and 
the second the later Miocene and Pliocene. This division differs 
from that of the right-hand column above only in putting the lower 
part of the Miocene in the lower division. 

Eocene formations appear in widely separated parts of North 
America (Fig. 471), though they do not appear at the surface over 
large areas. They include marine formations, brackish-water forma- 
tions (made in bays and estuaries), and land (lacustrine and subaé- 
rial) formations. Themarineand brackish-water beds areconfined to 
the borders of the continent, while the terrestrial deposits are found 
in the Great Plains and farther west. Many of the formations are 
not indurated, but locally they are even metamorphosed. 

The eastern coast.! Eocene formations appear at the surface 


1 Dall, 18th Ann. Rept., U. S. Geol. Surv., Pt. IT. 
556 





Fig. 471. Map showing the distribution of the Eocene formations in North 
America. The conventions are.the same as in former maps. 


in an interrupted belt near the coast from New Jersey to Texas. 
Their structure is similar to that of the Cretaceous system, upon 


558 EOCENE AND OLIGOCENE PERIODS 


which they are unconformable (Fig. 446). Clays, sands, and green- 
sand marls are the most common materials of the system, and the 
conditions of sedimentation were much as in the preceding period. 
The system is thicker (1,700 feet maximum) in the Gulf region 
than on the Atlantic coast. It contains much lignitic matter in 
places, showing that marine conditions were not uninterrupted. 
In Texas, gypsiferous and saliferous sediments recur at various 
horizons, though most of the beds are of marine origin, and there are 


Lecenpb 


MARINE 


I FRESHWATER 7 
































| / 
tty, 





i 


/ 


——S 
SS 
—— 
——- 
— 
eee 
———s 
==, 
eran se 
— 
a 
a 
—— 
= 
= 
— 
= 

= 

= 

e 

2 

° 

' 





| 














| 


—— 





Fig. 472. Map showing supposed 
distribution of land and water on the 
Pacific coast of the United States 
during the Eocene period. (Ralph 
Arnold.) 


numerous local unconformities in 
the system, suggesting repeated 
changes in the conditions of sedi- 
mentation. 

The Pacific coast. Marine and 
brackish-water beds. Marine 
Eocene formations are widespread 
west of the Sierra and Cascade 
ranges (Fig. 472), and have con- 
siderable development in Alaska. 
Throughout Washington and 
Oregon and in parts of California, 
the Eocene is unconformable on 
the Cretaceous (or Shastan), but 
in much of California it is con- 
formable on the Chico, the plane 
between the two being defined by 
fossils. These relations suggest 
that just before the Eocene, all 
of Washington, most of Oregon, 
and parts of the coastal region of 
California were land, over which 
the sea advanced later. The 
rocks are mostly clastic, sandstone 
and shale predominating, but 
there are conglomerates, tuffs, and 
diatomaceous’ shales, the last 
thought to be a source of oil. In 
not a few places, marine beds are 
succeeded by brackish-water de- 
posits. 


1 Arnold, Jour. Geol., Vol. XVII. 


FORMATIONS AND PHYSICAL HISTORY 559 


By the beginning of the Eocene, the Puget Sound depression, 
perhaps to be correlated with the great valley of California and the 
Gulf of California, had begun to show itself. The lands east and 
west of the sound were high, but not mountainous; and the region 
of the sound was a great estuary, in and about which deposition was 
in progress. Some of the sediments accumulated in brackish water 
and on land, and resulted in the thick coal-bearing Puget series of 
Washington, the upper part of which is Oligocene or even Miocene. 
The series is said to contain 125 beds of coal thick enough to attract 
prospectors. Most of the workable coal is in its lower part. The 
area of deposition extended south into Oregon, and east toward the 
Blue Mountains of that state. The system has an estimated thick- 
ness of 10,000 to 12,000 feet in southern Oregon, and but little less 
in southern California. 

British Columbia appears to have been land during the period, 
but Eocene beds, much disturbed ( Kenai series), have been recog- 
nized in Alaska, where they are coal-bearing in places. 

After the Eocene there was a time of temporary elevation, 
erosion, and volcanic activity along the Pacific coast, with consider- 
able basaltic flows in Washington and Oregon. 

The western interior. The warpings, faultings, and the intru- 
sions and extrusions of lava which marked the close of the Meso- 
zoic era in the west appear to have developed lands which were 
relatively high, adjacent to tracts which were relatively low. The 
steep slopes of the mountain folds, fault scarps, and volcanic piles 
seem to have afforded the conditions for rapid erosion, while the 
adjacent lowlands furnished places of lodgment for much of the 
sediment. Some of it took the form of fans and alluvial plains, and 
some of it probably lodged in lake basins formed by warping and 
faulting, or by the obstruction of valleys by lava flows. The wind 
also made its contribution to the deposits of the time, and the 
Eocene system contains much pyroclastic material. The result 
was a combination of lacustrine, fluvial, pluvial, eolian, and vol- 
canic deposits. 

The sites of principal sedimentation shifted somewhat from 
time to time, and among the widely distributed deposits referred 
to this period there are great differences of age. Several more or 
less distinct stages of deposition have been made out, the distinc- 
tions being based partly on the superposition of the beds, and partly 

1 Willis, Tacoma folio, U. S. Geol. Surv. 


560 EOCENE AND OLIGOCENE PERIODS 


on their fossils. These several stages are not readily correlated 
with those of the coasts. 7 

1. Reference has been made (p. 539) to certain formations 
(Denver, Raton, Lance, etc.), formerly classed as Cretaceous, which 
probably should be regarded as early Eocene. Some of these beds 
are inseparable from the Fort Union formation (or series), commonly 
regarded as the oldest division of the Tertiary in the western inte- 
rior. During the Fort Union stage, there was an extensive area of 
aggradation in parts of North Dakota', Montana, and farther north. 

The Fort Union beds are clastic and are said to be locally 2,000 
feet or more thick. Parts of the formation may be lacustrine, but 
parts are subaérial ? as indicated by the abundance of leaves at 
many places. The Fort Union series contains much coal, including 
some that was formerly classed as Laramie. Eocene formations of 
similar age are found in Colorado (Telluride and Poison Canyon 
formations), New Mexico (part of the Puerco beds), and elsewhere. 

The sites of early Eocene deposition were finally shifted. In 
so far as the sedimentation was in lakes, the basins may have been 
filled or warped out of existence, and in so far as it was subaérial, 
deformative movements, or the progress of the gradational work of 
the streams, or both, may have been responsible for the shifting. 

2. During the next or Wasatch stage of the period, sediment 
was being deposited over parts of Utah, western Colorado, and 
Wyoming, and elsewhere. ‘The beds of this stage have a maximum 
thickness of several thousand feet, and are now 6,000 to 7,000 feet 
above the sea. About 77% of the fossils are of land life. 

3. The third recognized stage of the Eocene in this region is the 
Bridger, during which sedimentation was in progress in the Wind 
River basin north of the mountains of that name, and another, a 
little later, in the basin of the Green River, both in Wyoming, and 
in Utah south of the Uinta Mountains. It may have been during 
this stage, too, that the volcanic tuff (Sax Juan formation, 2,000 
feet and less thick) of southwestern Colorado was made. This 
last formation is of interest as an index of the vigor of volcanic 
action in this region. Beneath it, glacial drift was found in 1913 
by Professor Atwood. Its extent is undetermined, and it maybe 


1 Wilder. Jour. Geol., Vol. XII, p. 290, and Leonard, State. Geol. Surv. of 
North Dakota, Fifth Biennial Rept. 

* For criteria for distinguishing lacustrine and subaérial formations, see Davia 
Science, N. S., Vol. VI, p. 619, 1897, and Proc. Am. Acad. Arts and Sci., Vol. 
XXXV, p. 345, 1900. 


FORMATIONS AND PHYSICAL HISTORY 561 


older than the Bridger Stage. It is at the base of the Eocene in 
this locality, near Ridgway. 

4. The Uinta stage followed the Bridger. Deposition was then 
in progress in southeastern Utah and southwestern Colorado. 
Some of the Uinta beds now have an altitude of as much as 10,000 
feet, though they probably were deposited at a much lower level. 

The northwest. In the northwest there are Eocene formations 
not definitely correlated with the preceding stages. In northern 
Oregon, there are late Eocene beds of terrestrial origin (Clarno 
formation, largely volcanic tuff). In Washington, two thick sedi- 
mentary formations (the Swauk, early Eocene, 3,500-5,000 feet, 
below, and the Roslyn, 3,500 feet) of Eocene age and non-marine 
origin, are separated by 300-4,000 feet of basalt. The Payette 
formation of Idaho, said to have been accumulated in a lake formed 
by the damming of the upper basin of the Snake River by the early 
lava-flows of the Columbia River region,! is now referred to the 
Eocene. Eocene beds of terrestrial or volcanic origin are imper- 
fectly known in many other places west of the Rocky Mountains. 
The erosion of the Eocene has given rise locally to the topography 
characteristic of ‘‘Bad Lands.”’ 

General considerations. It has been customary to regard the 
Eocene and later periods as much shorter than those of the Paleo- 





Fig. 473. Section showing the structure of the Eocene in western Oregon. 
Eb, Eocene basalt; Ep (Pulaski foruation), and Ec (Coaledo formation), Eocene. 
Length of section about 20 miles. (Diller, Coos Bay, Ore., folio, U. S. Geol. Surv.) 





Fig. 474. Section a little south of the last, showing the relation of the Eocene 
(Ep, Pulaski formation) to the Cretaceous (Km, Myrtle formation). as, amphib- 
olite schist, and Ps, Quaternary marine sand. (Coos Bay folio, U. S. Geol. Surv.) 


zoic and Mesozoic; but this conclusion may be questioned. On 
the basis of thickness, the showing of the system is great, both on 
the Pacific coast, and in the western interior. Furthermore, any 
just estimate of the duration of the period must take account of the 


1 Lindgren and Drake, Nampa and Silver City, Idaho, folios, U. S. Geol. 
Surv., and Knowlton, Bull. 204, p. 110. 


562 EOCENE AND OLIGOCENE PERIODS 


great erosion which followed the post-Cretaceous deformation. On 
the physical side, therefore, there is no warrant for assuming that 
the period was short. The faunal developments of the period were 
such as to make great demands upon time, and it is not improbable 
that the period was as long as the average of those of the Paleozoic 
and Mesozoic eras. 

Such thicknesses of terrestrial sediment as occur in the Eocene 
of western North America, if they are really as great as reported, 
call for explanation. If the areas concerned were in process of 
more or less continuous warping, low areas going down as surround- 
ing lands went up, or if troughs or basins of deposition were pro- 
duced by faulting, the bottoms sinking while their surroundings rose, 
the conditions would perhaps be met. 

The relations of the Eocene beds of the western interior indicate 
that both the attitude and altitude of the surfaces in that part of 
the continent were very different from those which now exist. That 
region must have been much lower than now, and, locally and tem- 
porarily at least, without well-established drainage. The present 
mountains were certainly not so high as now, though considerable 
elevations and great relief doubtless existed. 


Close of the Period in North America 


The closing stages of the Eocene were marked by crustal move- 
ments in the west, resulting in considerable changes in geography. 
Some such movements had taken place during the period, as has 
been indicated; but the faulting and folding at its close were on a 
larger scale. ‘The result was the retreat of the sea along the Pacific 
coast, the development of new areas of high and low lands, and there- 
fore a shifting of the sites of rapid degradation and aggradation. 

Along the Atlantic and Gulf coasts the Miocene is in many 
places unconformable on the Eocene, and it was at the close of the 
Eocene (or perhaps during the Oligocene) that an island, now in- 
cluded in the peninsula of Florida, was formed. In the Carolinas, 
and in the western Gulf region, the conformity between the Eocene 
and Oligocene formations seems to preclude notable changes of 
geography along the coast in the southeastern part of the United 
States at the close of the Eocene. 


Foreign 
Europe. Considerable lakes, estuaries, and perhaps other 
areas of deposition remained over western Europe, at the close of 


FORMATIONS AND PHYSICAL HISTORY 563 


the Mesozoic era. Later, but still early in the Eocene, submergence 
set in, allowing the sea to cover considerable areas from which it 
had been excluded temporarily. In western and central Europe the 
maximum submergence of the Eocene seems to have been accom- 
plished by the middle of the period. Toward its close, the epiconti- 
nental waters of the northwestern part of the continent were again 
restricted. 

In the south, the Eocene sea spread much beyond the borders 
of the present Mediterranean, covering much of southern Europe 
and northern Africa. Eastward it joined the Indian Ocean, cut- 
ting off the southern peninsulas of Asia from the mainland to the 
north. A sound east of the Urals 
probably connected the Arctic Ocean 
with the expanded Mediterranean. 
Above this sea rose many islands, 
some of which corresponded in 
position to the Alps, Carpathians, 
Apennines, and Pyrenees. 

On the bottom of this great body 
of water, limestone was deposited ee hi on nummulitic 
Onean extensive scale: Much of tfimestone. 
it is made up almost wholly of the 
shells of nummulites (foraminifera, Fig. 475), and is found from 
one side of the Old World to the other. Since it is thick 
(locally several thousand feet) as well as widespread, the sea must 
have swarmed with foraminifera, and the period must have been 
long. In few other places are there indications of such great num- 
bers of organisms of one kind. Some idea of the deformative move- 
ments since the Eocene may be gained from the fact that the num- 
mulitic limestone occurs at elevations of more than 10,000 feet in 
the Alps, up to 16,000 feet in the Himalayas, and 20,000 feet in Tibet. 
In the Old World as well as in the New, the greater relief features of 
the present are post-Eocene. 

Other continents. Marine Eocene is known along the northern 
and western coasts of Africa, and in the Soudan, in South Australia, 
New Zealand, and Tasmania, and in various islands of the Pacific. 
The Tertiary formations of South America have not been closely 
correlated with those of other continents. There is marine Eocene 
along some parts of the western coast, in Patagonia! (Magellanian 

1 Hatcher, Am. Jour. Sci., Vol. IV, 1897, p. 334, and Vol. IX, 1900, p. 97. 





564 EOCENE AND OLIGOCENE PERIODS 


series) and probably elsewhere in Argentina, and along at least a 
part of the coast of Brazil! Non-marine beds occur in Patagonia. 
Eocene beds are extensive in the West Indies where limestone 
is the dominant rock. Formations of this age are said to occur up 
to elevations of 10,500 feet in Hayti.* It was formerly thought — 
that the Atlantic and Pacific oceans connected freely somewhere 
south of the United States during the early Tertiary, but the work 
of Hill renders it doubtful whether there were more than shallow 
and restricted connections in the Eocene, and whether there was con- 
nection of any sort later. | a 


General Geography of the Eocene 


The geography of the Eocene was very different from that of the 
present time, and the differences were perhaps even greater than has 
been aaioetcdl It has been conjectured that North America was 
connected with Asia on the west, by way of Alaska, and with Europe 
on the east, by way of Greenland and Iceland. Land seems to have 
failed of making a circuit in the high latitudes of the north only by 
the strait or sound east of the Urals. In the southern hemisphere, 
it has been surmised that Antarctica was greatly extended, con- 
necting with South America, Australia, and possibly with Africa, 
and that Africa and South America were connected across the 
Pacific from some earlier time until after the beginning of the 
Eocene. The basis for these conjectures is found in the distribution 
of life at that time, as shown by fossils. 

If these conjectured extensions of land were real, it will be seen 
that the division of land and water in the northern and southern 
hemispheres was far less unequal than now, that the land was massed 
in high latitudes to a great extent, and that tropical seas were more 
extensive. If extensive polar lands were the cause of glacial periods, 
as some have thought, the geographic conditions of the Eocene were 
. favorable in the extreme for glaciation, if the relaticns sketched 
above were the real ones. In spite of this, the climate of the period 
seems to have been genial, and less markedly zonal than now. 

Close of the Eocene. During the later part of this period, and 


1 Branner, Bull. Geol. Soc. Am., Vol. XIII, and Stone Reefs of Brazil, Mus. 
of Comp. Zodél., Bull. 44, pp. 27-53. 

2 Hill, Geological History of the Isthmus of Panama and Portions of Costa 
Rica. Bull. Mus. of Comp. Zodél., Cambridge, 1898. Also J. P. Smith, Science, 
Vol. 30, 19090, p. 348. 


LIFE 565 


at its close, there were some notable deformations in southern 
Europe. The initiation of the Pyrenees, and of some of the moun- 
tains farther east, are among the larger disturbances assigned to 
this time. The greater deformations which expressed themselves in 
the mountains of Southern Europe were post-Eocene, and most of 
them considerably later than the close of the Eocene. 


LIFE 


Transition from the Mesozoic. Four salient features marked 
the transition of life from the Mesozoic to the Cenozoic: (1) among 
marine animals, nearly all Cretaceous species were replaced by new 
ones; (2) so many species of land plants lived on as to make it diffi- | 
cult to separate the Mesozoic from the Cenozoic; (3) the great 
saurlians almost disappeared, and most other reptiles showed pro- 
found changes; and (4) mammals appeared in force, and promptly 
took a leading place. 

The great change in the epicontinental marine life was due, 
no doubt, to the withdrawal of the sea from the continent, and the 
great restriction of the area of shallow water. The increase of the 
land and the establishment of new land connections may well have 
caused the existing vegetation to spread and flourish, if the climate 
remained congenial; but the land faunas did not respond in like 
manner. 

It is an open question whether the Eocene mammals of North 
America and Eurasia descended from the primitive types of mam- 
mals which lived in these continents earlier, or whether they were 
immigrants. Satisfactory evidence of their descent from the early 
(non-placental) mammals is wanting, and the suddenness of their 
appearance in great numbers suggests invasion from some other 
quarter. The deformative movements which inaugurated the 
Eocene period quite certainly made new land connections, and fur- 
nished the conditions for an invasion, if. mammals, developed else- 
where, were awaiting the opportunity. 

Perhaps the rise of mammals caused the downfall of the reptiles. 
The habit of bringing forth relatively mature offspring, and of nour- 
ishing and protecting them, gave the mammals an immense advan- | 
tage, to wich were added superior agility and higher brain power. 
It would not be surprising, therefore, if the rise of mammals drove 
the clumsy, small-brained reptiles either to extinction, or to the 
assumption of new and smaller forms. 


566 EOCENE AND OLIGOCENE PERIODS 


Vegetation 


In plant history the Eocene was not the dawn of the recent, 
for the change from medieval to modern plants took place in the 
Comanchean period. The Eocene did not even mark any radical 
innovation. There was, however, much progress toward living 
species, and toward present adaptations of plants to climate, soil, 
and topography, and to each other. 

Among the plants of the earliest known Tertiary flora of Europe 
were oaks like those of the present high lands of warm temperate 
zones. With them were willows, chestnuts, laurels, etc., which 
have been likened to the flora of southern Japan. The flora of the 
Denver beds (p. 539), contains figs, poplars, laurels, magnolias, and 
many ferns. The early Eocene flora of southern Canada included 
similar forms, together with oaks, beeches, etc., a flora indicating a 
temperate climate. 

The Middle Eocene of England records a flora ‘‘the most 
tropical in general aspect which has yet been studied in the north- 
ern hemisphere,” ? while a later flora ‘‘suggests a comparison of its 
climate and forests with those of the Malay Archipelago and tropical 
America.’’ The mid-Eocene of America in temperate latitudes 
contains palms and bananas, mingled with many other trees of 
similar climatic significance. The Eocene flora of Alaska indicates 
a climate comparable to that of Southern California and Florida. 
This flora shows a curious commingling of Jurassic and Cretaceous 
types (cycads), with angiosperms. A similar flora in the island of 
Saghalien indicates land connection between Alaska and Asia.’ 


Early Eocene Mammals 


The mammals of the earliest Eocene included several poorly 
differentiated groups, in which existing orders were foreshadowed 
rather than represented. The herbivores were foreshadowed by the 
Condylarthra, and the carnivores by the Creodonta; but the twogroups 
were not sharply differentiated. Both were five-toed plantigrades, 
whose phalanges had horny coverings that were neither hoofs nor 
claws. Edentates, insectivores, rodents, and lemuroids seem to have 
been represented or foreshadowed. Evolution was so rapid that 
before the close of the Eocene, most existing groups of mammals 
were well defined (p. 686). None of the present genera, however, 


1 Geikie, Textbook of Geology, 3d ed., p. 974. 
2 Hollick, Am. Jour. Sci., Vol. 31, 1911, pp. 327-30. 


LIFE 567 


existed then. In general, the mammalian faunas of the Eocene 
of North America were closely similar to those of western Europe, 
while during the Middle and Late Eocene there seems to have 
been faunal separation between these continents.! 

Main herbivore line. While the condylarths and creodonts 
were near each other at the beginning of the period, the hoofed 
herbivores and the clawed carnivores developed from them soon 
became distinct. The condylarths (Fig. 476) were small generalized 





Fig. 476. A primitive ungulate or condylarth of the Wasatch epoch; Phenaco- 
dus primevus Cope, about 1/13 natural size (about the size of a tapir), from Big 
Horn basin, Wyoming. (Cope.) 


forms with five toes and forty-four teeth, not yet developed into true 
herbivores. Condylarths did not live beyond the Eocene, but one 
branch adapted to forests and marshes seems to have diverged early, 
and perhaps to have given rise later to the ungulates. In the course 
of the period many of them became fitted for life on grassy plains. 
To this end, the flat, heavy, palmate form of foot adapted to marshes, 
gave place gradually to the light, springy, digitate form, adapted to 
a quick start and swift flight. At the same time hard hoofs, and 
grinding teeth were developed. The evolution of hoofs and grind- 
ing teeth has been thought to be connected with the prevalence of 
grassy plains, the firm turf of which«is in contrast with the soft soil 


1 For references to important literature on the American Tertiary Mammalia, . 
see the authors’ larger work, Vol. III, p. 228. See also Osborn, Bull. 361, U.S. 
Geol. Surv., and The Age of Mammals; and Scott, A History of Land Mam- 
mals in the Western Hemisphere. 


568 EOCENE AND OLIGOCENE PERIODS 


of forest and marsh. Forests perhaps helped to preserve a section 
of the evolving order in its more primitive form. 

Back of these influences lay the physical conditions that pro- 
moted them. In western America, where the evolution is best 
known, the lakes and rivers were undergoing changes. As they 
shrank or shifted, they left behind them borders of grassy or sedgy 
ground which, on fuller drainage, may have become prairies. Such 
changes were suited to theevolution of herbivorous prairie life, and. 
this in turn must have invited predaceous animals. If these con- 
siderations are valid, the prime factors in the evolution of the un- 
gulates were (1) an undifferentiated plastic animal group susceptible 
of modification; (2) a plant group (grasses and fodder-furnishing 
angiosperms) affording appropriate food for the new type; and (3) 
the shrinkage and shifting of lakes, marshes, and lodgment plains, 
and the drying up of the plains of the continent, resulting in prairies 
whose hard turf favored the development of foot and limb modifica- 
tion in the interest of speed. 

The era of bulk and heavy armor, such as had been possessed 
by the reptiles, had passed, and an era of agility and dexterity had 
begun. No small factor in this progress was the increase in intel- — 
ligence indicated by the larger brains. The lighter and more agile 
frame was accompanied by the development of smaller, simpler, 
but more effective weapons of attack and defence. Nevertheless 
size continued to be important, and some species in almost every 
sub-order reached and passed the limit of bulk-advantage, and then 
declined. 

In the course of the early evolution strange forms appeared, 
and soon became extinct. Among them were the Dinocerata (Fig. 
477), grotesque monsters whose skulls were armed with three pairs 
of protuberances, perhaps horn cores, and a pair of enormous 
canine teeth or tusks projecting below (at least in the male), and 
an extravagant attempt at armature on both upper and nether sides. 
Their brains were singularly small for such ponderous bodies. In 
them, brute mass and low brain-power seem to have reached their 
climax among mammals. 

Divergence of ungulates into odd- and even-toed. Early in 
the Eocene, hoofed animals began to diverge into odd-toed (peris- 
sodactyls) and even-toed (artiodactyls) types. In the former, the 
main line of support is in the axis of the middle toe; in the latter, 
between the third and fourth toes. In the course of time the lateral — 


LIFE 569 





re) 
= 
~ = >a 
é kL 


Fig. 477. Dinoceras mirabile, restoration of skeleton by Marsh; about 13 feet 
long. Middle Eocene, Wyoming. 


toes fell out of use and were atrophied. The first class reached its 
extreme type at length in the horse, and the second in our cloven- 
hoofed cattle; but these perfected types were not attained in the 
‘ocene. 

The horse has become a classic example of evolution. The 


=e ee es Oe © 





Fig. 478. An early ancestor of the horse family, Hyracotherium (Protorohip pus) 
venticolum, from the Lower Eocene (Wind River formation) of Wyoming; about 4 
natural size. (Cope.) 


earliest recognized form was the Hyracotherium (Fig. 478), which 
resembled the horse but little. The Orohippus (Epihippus) repre- 
sented a greater advance. It was four-toed (three functional) in 


570 EOCENE AND OLIGOCENE PERIODS 


front, and three-toed behind. It had about the size of a small dog, 
and was as much canine as equine in appearance. ‘True horses did 
not appear till the Pliocene. 

Artiodactyls emerged from their generalized beginnings more 
slowly. Suina (pigs, peccaries, hippopotamuses) were represented 
in the Bridger epoch by a small hog. Strangely enough, the camel 
family seems to have had its beginning in America in the middle and 
later Eocene, and to have flourished here until the Pliocene. Then, 





Fig. 479. Mounted skeleton of Patriofelis, a Creodont from the Middle Eocene 
of Wyoming; 1/18 natural size. (Osborn.) 


having previously sent a branch to South America to evolve into 
llamas and vicunas, and another into the Old World to become the 
present camels, the tribe died out in its primitive home. 

Carnivore line. It has been thought by some paleontologists 
that the creodonts were more primitive than the condylarths, and 
that the latter diverged from the former, as also the edentates and 
the rodents. If this is so, it gives the creodonts the central position 
among the primitive mammals. They lived throughout the period 
and into the next, gradually giving way to their own more progres- 
sive descendants. Toward the end of the period, modern types 
began to emerge definitely from the ancestral forms. Primitive 
representatives of the dog family appeared in Europe late in the 
period. Scott states that “clawed mammals long antedated the 


LIFE 571 


hoofed types, and that the latter arose, either once or at several 
separate times, from the former.! 

Edentates, rodents, and insectivores. The similarity of the 
ancestral edentates to the condylarths and creodonts of the earliest 
Eocene seems to imply that the three orders had but recently di- 
verged from common ancestors. By the middle of the period, 
rodents became a notable element in the fauna. The squirrel 
appeared in Europe in the latter part of the period. Even to-day, 
the rodents retain many primitive characters, and since the Miocene 
have undergone few radical changes. Their derivation is not yet 
determined. Most 
living families of im- 
sectivores can be traced 
back to the Eocene. 
They still retain many 
primitive characters, 
and are the least altered 
of the great mamma- 
lian branches. 

Non-placentalmam- 
mals. During the 


period, opossums ap- Fig. 480. The skull and jaw of a large Eocene 
peared in both hemi- rodent, Tillotheriuwm fodiens Marsh, from the Bridger 
formation, Wyoming; about 1/6 natural size. 








spheres. They retained 
this wide distribution until the Miocene, when they disappeared 
from Europe, but they have persisted in North and South America 
to the present. It isa singular fact that the monotremes, the lowest 
of the mammals, are not known until after the Tertiary. 

The primates. No traces of apes have been found in the Eocene, 
but lemuroids appeared in the Wasatch epoch in America, and in a 
similar horizon in Europe. ‘This is the more notable, as the lemurs 
are now confined to Madagascar, Africa, and southern Asia. They 
have many affinities with the insectivores, and were possibly derived 
from them. Apes probably descended from the early lemuroids. 

Mammals go down to sea. Some mammals took to the sea by 
choice or necessity, as land reptiles did before them. Thus arose 
cetaceans (whales, dolphins, porpoises), sirenians (manatees, du- 
gongs), and pinnipeds (seals, sea-lions). In parts of Alabama, verte- 
bre of primitive whales (Zeuglodons) were originally so abundant 

1A History of Land Mammals in the Western Hemisphere. 


572 EOCENE AND OLIGOCENE PERIODS 


as to attract popular attention, and call forth legends of divers 
catastrophes. 

Birds. Fossils of many types of birds, such as gulls, herons, 
eagles, owls, quails, plovers, and flightless birds of great size, show 
great deployment of this class. 

Reptiles and’ amphibians. One of the greatest contrasts in 
geological history is found in comparing the size, power, and multi- 
tude of the Cretaceous land reptiles with those of the Eocene. Of 
the great saurians, only a few lived on into the early Eocene. Land — 
reptiles seem to have become rare early in the period, though there 





a, 
Fig. 481. EOCENE FoRAMINIFERA. @, Nodosaria bacillum Defrance; 6, N. 
communis (d’Orbigny); c, Anomalina ammonoides (Reuss); d, Cristellaria gibba 
d’Orbigny; e, C. radiata (Bornemann); f, g, and h, Globigerina bulloides d’Orbigny; 
i, Vaginalina legumen (Linné); j7, Discorbina turbo (d’Orbigny); k, Truncatulina 
jobaiula (Walker and Jacob); 1, Textularia subangulata (d’Orbigny). Magnified 
8 to 40 times. (Maryland Geol. Surv.) 


were turtles on the land and in the sea, and some of them attained 
large size. ‘There were crocodiles which belonged about equally to 
land and water; also snakes, some of them large. Amphibians 
were present, but apparently not abundant. 

Insect life. There has been little important change in the in- 
sect world since the beginning of the Cenozoic. Few new families 
have appeared, though genera and species have changed, 


LIFE aaa 





a-h, Gastropods: a, Fusus (?) interstriatus 


Fig. 482. EocENE MOoLtusks. 
Heilprin; b, Mitra potomacensis Clark and Martin; c, Pleurotoma tysoni Clark and 


Martin; d, P. potomacensis Clark and Martin; e, Scala potomacensis Clark and 
Martin; f, Tornatellea bella Conrad; g, Turritella mortoni Conrad; h, Lunatia mary- 
landica Conrad. i-w, Pelecypods: 1, Glycimeris idoneus (Conrad); j, Dosiniopis 


Continued at the bottom of p. 574. 


574 EOCENE AND OLIGOCENE PERIODS 


Marine Life 


The name Eocene (dawn of the recent) was meant to imply the 
presence of less than 5% of living species among the marine inverte- 
brates of the period; but most existing orders, families, and genera 
were established. The changes of later times are considerable, and 
are valuable as criteria for correlation, climatic changes, migrations, 
etc., but they are not profound biological transformations. They 
are in striking contrast with the radical and rapid evolution of the 
mammals. 

Geologically, the most striking feature of the marine Eocene life 
was the extraordinary abundance and size of the foraminifers 
(Fig. 481). Most types of marine invertebrates had assumed their 
modern forms. 

The American Eocene faunas were rather pronouncedly pro- 
vincial, though some species have a rather wide range. So pro- 
nounced is their provincial character that much difficulty is experi- 
enced in making correlations between formations along different 
parts of the Atlantic and Gulf coasts, and greater difficulties arise 
in regions more widely separated. The variations are, however, 
variations of detail, not of broad features. 

The marine fauna of the Pacific coast,! and the flora as far north 
as Puget Sound, indicate a subtropical climate. 


OLIGOCENE FORMATIONS 


North America. Formations corresponding to the Oligocene 
of Europe have not been differentiated completely in North Amer- 
ica;? but certain formations along the Atlantic and Gulf coasts, 
formerly classed as late Eocene or early Miocene, may be regarded 
as equivalent-:to some part of the Oligocene of Europe. In the 
Gulf region the Vicksburg (below) and Grand Gulf formations of 
Alabama, Mississippi, and Louisiana, and the Fayette formation 
of Texas, belong to this category. The early Oligocene is repre- 
sented generously about the Caribbean sea, where its association 


lenticularis (Rogers); k and 1, Venaricardia marylandica Clark and Martin; m and 
n, Corbula aldrichi Meyer; 0 and p, Protocardia levis Conrad; q, Ostrea compresst- 
rostra Say; r, Modiolus alabamensis Aldrich; s, Lucina aquiana Clark; t, Leda 
parilis (Conrad); «, Crassatellites aleformis (Conrad); v, Nucula ovula Lea; w, Pecten 
choctawensis Aldrich. (Maryland Geol. Surv.) 


1 Arnold, Jour. Geol., Vol. XVII, p. 509, and Knowlton, Tacoma, Wash., Folio. 
2 Dall, 18th Ann, Rept., U. S, Geol. Surv., Pt. IT. 


LIFE 575 


with the Eocene is close,! and its separation from the Miocene 
distinct. This is in keeping with the phenomena of the Gulf 
States. Limestone is the dominant rock in the Antillean region. 
The Oligocene stage is also recognized among the terrestrial 
deposits of the western part of the continent. The White River 
formation, now classed as Oligocene, occupies an extensive area in 
northeastern Colorado, southwestern Wyoming, western Nebraska 
(Brule and Chadron formations), and South Dakota, and perhaps 
in Kansas. In the light of present knowledge, it seems probable 
that all phases of land aggradation, lacustrine, fluvial, and eolian, 





Fig. 483. Chimney Rock, a detail in the Bad Lands of the White River coun- 
try. The base of the column is Brule clay. (Darton, U.S. Geol. Surv.) 

are represented in this formation.” Even thin beds and lenses of 
limestone and volcanic ash enter into it. The formation is said 
originally to have covered most of the Black Hills region, and pos- 
sibly all of it.2 Remnants are found up to elevations of more than 
6,000 feet, and the highest points of the hills are but little higher. 
The Florissant beds in South Park, Colorado, consisting largely of 
volcanic ash, and famous for their fossil insects, are classed as 
Oligocene. So also are some of the beds of the John Day Basin of 
Oregon, unconformable above the Eocene. Marine Oligocene beds 


1 Hill, Geology and Physical Geography of Jamaica, and Geological History 
of the Isthmus of Panama and portions of Costa Rica. Bull., Mus. Comp. Zodl., 
Vols. XXVIII and XXXIV respectively. 

2Fraas, Science, Vol. XIV, N. S., p. 212, and Matthew, Am. Nat., Vol. 
XXXIITI, p. 403, 1899. 

3 Darton, 19th Ann. Rept., U. S. Geol. Surv., Pt. IV; 21st Ann, Rept., U. S. 
Geol. Surv., IT. 


576 EOCENE AND OLIGOCENE PERIODS 


are found on the Pacific coast, but the record of the period here is 
found chiefly in the unconformity between the Eocene and the 
Miocene. 

Considerable geographic changes occurred during the Oligocene, 
or at its close, especially in the Gulf and Caribbean regions, where 
the Oligocene (early Oligocene) is commonly conformable on the 
Eocene, and unconformable beneath the Miocene. 





Fig. 484. Oligocene Bad Lands of South Dakota. (Williston.) 


Europe. Toward the close of the Eocene, the epicontinental 
sea of northern Europe was greatly restricted, but considerable 
areas stood so near sea-level that slight changes served greatly to 
diminish or extend the epicontinental waters. 

The oldest Oligocene deposits of central and western Europe 
are largely of terrestrial, fresh- and brackish-water origin. Local 
deposits of salt, gypsum, and coal are suggestive of the physical 
conditions at various times and places. The Oligocene of southern 
Europe is chiefly marine. 

In Europe, as in North America, there were considerable igneous 
eruptions during the Tertiary, and especially during the Oligocene. 
Between eruptions, vegetation grew in marshes and shallow lakes 


LIFE 


and over the surtace of the lava. 


577 


The substance of this vegetation 


is locally (Faroe Islands and Iceland) preserved in the form of coal 


between the lava beds. 


Amber. One of the peculiar accessories in the Lower Oligocene 


is the amber of northern Ger- 
many, principally in the vicinity 
of Konigsberg. While amber in 
small quantities is found in Sicily 
and a few other places, that of 
the Baltic region is more abund- 
ant than that of any other part 
of the earth, so far as now known. 
Amber is fossilized resin, ap- 
parently from certain varieties of 
coniferous trees. Its original 
position in the Baltic region ap- 
pears to be in ‘certain beds of a 
clayey nature, but parts of. this 
formation have been worn by the 
waves, and the amber distributed. 
Some of that which finds its way 
into commerce is picked up on the 
Baltic shore, while some is taken 
from the beds in which it was 
originally entombed. One of the 
interesting features of the amber 
is the fact that it contains numer- 
ous insects. 


was soft, and to, have. become 
completely immersed in’ it, and 
perfectly preserved. About 2,000 
species have been found thus 
embedded. 

Considerable deformative 
movements made themselves felt 


They seem’ to:have | 
alighted upon the resin while it . 






Lecenpn 









MARINE 


cn =p 
FREsHWwATER -— 



























| 
| ; 
| 


| 
) 
al 
= 


4 


Ui 
Pi 
od 
au 





= 


So 
u 
O LIGOCENE 
TRadph Arn ela, i404 i NX 


Fig. 485. Map showing supposed 
distribution of land and water on 
the Pacific coast of the United States 
during the Oligocene epoch. (Ralph 
Arnold.) 
































in southern Europe at or about the close of the Oligocene, as in the 
Balkan and Carpathian Mountains.’ 


Other continents. 
1 Willis. 


In other continents, the Oligocene has not 
Carnegie Institution Year Book 4, 1905. 


578 EOCENE AND OLIGOCENE PERIODS 


been generally differentiated, but it is known in northern Africa 
and in Patagonia,! where it is partly marine and partly non-marine. 


OLIGOCENE LIFE 


Vegetation. The forests of the Oligocene were similar to those 
of the Eocene, especially in Europe, where palms continued to be 
abundant and varied, growing even in north Germany. The Floris- 
sant beds of Colorado contain a variety of angiosperms, representa- 
tive of orders now found in the latitude of the middle and southern 
states. 

Land animals. All species of imsects in the Florissant beds 
(over 700) are extinct. This indicates that although the types had 





Fig. 486. Titanotherium validum Marsh, , photograph of a mounted specimen 
in the Carnegie Museum. (Holland.) 


become modern, the species continued to change with relative 
rapidity. Fish fossils are abundant in the same beds. 

Mammals continued their rapid evolution. The Carnivora 
came into clear definition, and were represented in the White River 
beds by ancestral dogs, cats, raccoons, and weasels, while some creo- 

1 Hatcher, Geol. Mag., 1902, p. 136. 


LIFE 579 


donts remained. Rodents were represented by squirrels, beavers, 
pocket-gophers, rabbits, and mice. Among perissodactyls, the 
rapidly developing horse family was represented by Mesohippus 
and Anchippus. The rhinoceros tribe had deployed into three 
branches, one a lowland form, ancestral to the existing family, one 
aquatic, and a third an upland running form. The tribe had a cos- 
mopodlitan range. 

An erratic branch (the ¢titanotheres) of the odd-toed ungulates 
which arose late in the Eocene reached its climax in the Oligo- 





Fig. 487. An interpretation of the elotheres, or giant pigs, of the White River 
epoch, drawn by Charles R. Knight. (From drawing in American Museum of Natural 
History. Copyrighted by the Museum.) 


cene (White River), and then disappeared. Its representatives 
were distinguished by a long, depressed skull, armed with a pair of 
horns near the end of the nose (Fig. 486). They reached some 
fourteen feet in length and ten in height. They were American and 
apparently rather local. Another odd type was the elothere, which 


580 EOCENE AND OLIGOCENE PERIODS 


appeared in North America in the White River stage, and con- 
tinued into the Miocene. An interpretation of their general 
appearance is shown in Fig. 487. Artiodactyls were prominent, 
represented by various extinct forms, and by ancestral peccaries, 
camels, ruminants, and other forms. 

Marine life. The fauna of the Oligocene on the Atlantic coast 
of North America has the same general aspect as that of the Eocene. 
Later, however, provincialism became pronounced. By this time, 
the foraminifers had declined greatly, and the fauna was over- | 
whelmingly molluscan. On the Pacific coast, the Oligocene fauna 
shows closer relation to the Miocene fauna than to the Eocene, and 
suggests a climate intermediate between the climates of those 
periods. “ 


CHAPTER XXVII 
THE MIOCENE PERIOD ! 
FORMATIONS AND PHYSICAL HISTORY 


. The geography of the North American continent during the 
Miocene period was similar to that of the Eocene. The slight 
emergence of the coastal borders after the Eocene (or early Oligo- 
cene) was followed by a slight submergence of the same regions 
during the Miocene. In the western interior, terrestrial aggrada- 
tion of all phases continued, but the sites of principal deposition 
differed somewhat from those of the preceding period. 

The Atlantic coast. In its surface distribution, the Miocene 
sustains the same relation to the Eocene that the latter does to 
the Cretaceous (Fig. 446), though in places the Miocene overlaps 
the Eocene, completely concealing it. There is generally a slight 
unconformity at the base of the Miocene. Like the other forma- 
tions of the Coastal Plain, the beds dip seaward and are concealed 
by younger beds some distance to landward from the present shore. 
The system originally extended inland far beyond its present border, 
as shown by numerous outliers. 

The Miocene of the Atlantic coast is composed chiefly of un-: 
consolidated sand and clay. In places there is shell marl, and local- 
ly beds of diatomaceous earth of such thickness (30 or 40 feet) as to 
be valuable commercially. At the north, the Miocene has a thick- 
ness of 700 feet, but it thins southward. The Miocene of this coast 
is generally called the Chesapeake formation. It was formerly 
regarded as Upper Miocene, the former Lower Miocene being now 
classed as Oligocene. ‘The fauna of the Chesapeake series has been 
interpreted to indicate a climate somewhat cooler than that which 
had preceded. 

_ The Gulf coast. The Miocene of the Gulf coast is rather thin, 
and sustains the same general relations to older formations as that 

* Dall and Harris, Bull. 84, U.S. Geol. Surv., and Dall, 18th Ann. Rept., U.S. 
Geol. ‘Surv. % Pt. dl. 

581 


582 THE MIOCENE PERIOD 


\ 


Jonzecnnpteeeee 





Fig. 488. Map showing the distribution of the Miocene formations in North 
America. Conventions as in preceding maps. 


of the Atlantic, except that it is not known to be so generally uncon- 
formable on tormations below. In Florida, Miocene limestone has 


FORMATIONS AND PHYSICAL HISTORY 583 


been changed locally to lime phosphate.! The alteration appears 
to have been effected through organic matter, especially the animal 
excrements accumulated about bird, seal, and perhaps other rook- 
erles. The organic matter furnished the phosphoric acid, which, 
carried down in solution, changed the carbonate of lime to the phos- 
phate. The phosphate is used extensively as a fertilizer. In 
Texas part of the Miocene is non-marine. Much of the oil of Texas 
and Louisiana comes from dolomized limestone which is probably 
Miocene.’ | 

The Pacific coast. At the beginning of the period, the sea 
encroached upon the Pacific coast, covering considerable areas 
which were .land during the Oligocene. It flooded the southern 
part of the central valley of California early in the period, and 
later the northern part as well. At about the beginning of the 
period, faulting seems to have affected considerable parts of Cali- 
fornia, and some of the planes of movement at that time have 
served as planes of movement since. This was the time of the 
first definitely recognized movement along the great earthquake 
rift of California. ‘Though subsidence was the rule in central and 
southern California, local fault-blocks seem to have had notable 
elevation. 

The Miocene history of the Pacific coast is divided into two 
somewhat distinct epochs, separated by diastrophism and vulcan- 
ism. During the first epoch, besides clastic formations and vol- 
canic ash, there is a formation (Monterey) containing much diato- 
maceous material which is an important source of oil. The amount 
of siliceous material ascribed to diatoms is prodigious, and seems 
credible only when the extraordinary rate of reproduction of diatoms 
is recalled. It has been estimated that a million individuals might 
come from one, in the course of a month. If this is the fact, it is 
perhaps not strange that large amounts of siliceous material accumu- 
lated where conditions favored. 

After the early Miocene there were extensive igneous eruptions 
in eastern Washington, Oregon, and the Coast ranges of California. 
South of San Francisco, this was the time of the last important 


1 Penrose, Bull. 46, U. S. Geol. Surv. 

2 Hayes, Bull. 213, U. S. Geol. Surv., p. 346. 

3 Arnold, Ralph, Jour. Geol., Vol. XVII. 

4 Eldridge, Bull. 213, U. S. Geol. Surv.; Arnold and Anderson, Bull. 322, U. S. 
Geol. Surv., 


584 THE MIOCENE PERIOD 


eruptions in the Coast ranges, though farther north vulcanism con- 
tinued later. The igneous eruptions were accompanied by diastro- 
phism, which consisted in the readjustment of fault-blocks and folds 
throughout the Pacific coast region. Even high mountains were 





Leaennd Lecenp 







Marine 


HN FResHwaTeR F= 


Marine 


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4 iy 
















































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| Lower Miocene : 


Ralph Arnola, 1904 





Fig. 490 

Fig. 489. Map showing supposed distribution of land and water on the Pacific 
coast during the early Miocene period. (Ralph Arnold.) 

Fig. 490. Map showing supposed distribution of land and water on the Pacific 
coast during the late Miocene period. (Ralph Arnold.) 


developed locally, as shown by the coarseness of the sediments 
which followed. 

The diastrophism resulted in the extension of the sea, for the 
Upper Miocene is more widespread than the lower. If a two-fold 


FORMATIONS AND PHYSICAL HISTORY 585 


division of the Tertiary were adopted, the earlier part of the Miocene 
should go with the early Tertiary, and the jater part with the late 
Tertiary. The marine part of the system has great thickness, the 
Lower Miocene having a maximum thickness of some 8,000 feet, and 
the Upper hardly less. | | 

By the end of the period, the peneplanation of the Klamath and 
Sierra Nevada Mountains seems to have approached completion. 





Fig. 491. Contorted beds of Monterey shale. Mouth of Vaquero Creek, Cal. 
(Lippincott, U. S. Geol. Surv.) 


Much of the material eroded from them had been deposited in the 
central valley of northern California, making the thick Miocene 
beds of that valley. 

In western Oregon, Miocene (Empire) beds a few hundred feet 
thick, containing volcanic ash, rest unconformably on the deformed 
and eroded Eocene. In British Columbia, there ‘are both clastic 
and volcanic rocks referred to this period. 

The Miocene of the western coast has not the simple structure 
of the corresponding beds along the Atlantic and Gulf coasts. 
The strata have been deformed so as to stand at high angles (Fig. 
491) in many places, and locally (Mount Diablo range) they have 


586 THE MIOCENE PERIOD 


been folded, and the folds overturned so that Cretaceous and Eocene 
formations overlie the Miocene. 

Non-marine deposits. In the northern part of the central val- 
ley of California there are deposits of estuarine, lacustrine, and 
probably subaérial origin (Jone formation) partly contemporaneous 
with the early Miocene marine beds farther south. They consist 
of the common sorts of clastic sediments, with some coal, iron, etc. 










PST Tes Sb ert 
rte i te 


HHT 





Fig. 492. Section showing the structure and relations of the Miocene system in 
the San Luis Obispo region of southern California. Js/, San Luis formation, 
Jurassic; Nm, Monterey shale, Miocene; N7rt, rhyolite tuff; Np, Pismo formation, 
Miocene (?); Vpr, Paso Robles formation, Pliocene; Pal, recent alluvium, etc. 


Along the east side of this valley, auriferous gravels,! brought down 
by streams from the Sierras, were being deposited during at least a 
part of the period. These gravels seem to have been laid down ona 
surface of slight relief, interpreted as a peneplain.? The Sierra 
Mountains are thought to have been at least 4,000 feet lower than 
now when these gravels were deposited. 

Non-marine Miocene beds are rather widespread in south- 
eastern California and Oregon, reaching great thicknesses at some 
points in the vicinity of the goth parallel. They include clastic 
sediments, volcanic debris, infusorial earths, and fresh-water lime- 
stones. 

Farther east, on the western part of the Great Plains, the deposi- 
tion of the White River beds may have continued for a time after the 
beginning of the Miocene. Late in the period, aggradation seems to 
have been renewed in the same general area, and the Loup Fork 
formation, thin but extensive, was spread over great areas from 
South Dakota to Mexico. The lacustrine phases of this formation 
are probably less extensive than the subaérial.* Like the White 
River formation, the Loup Fork beds have been eroded into ‘‘bad- 
land” topography (Figs. 68 and 69). 

Turner, 14th Ann. Rept., U. S. Geol. Surv., 1894; Lindgren, Jour. Geol., 
Vol. IV, 1896, pp. 881-906; Diller, Jour. Geol., Vol. II, pp. 32-54. See also folios 
of the Gold Belt of Calif., U. S. Geol. Surv. 


2 Diller, Jour. Geol., Vol. II, pp. 33-54. 
’ Haworth, Univ. Geol. Surv. of Kan., Vol. II, p. 281, 


FORMATIONS AND PHYSICAL HISTORY 587 


Non-marine deposits, largely of volcanic material, occur in 
British Columbia between the Coast and Gold ranges. Miocene 
deposits are known in Alaska, but erosion rather than deposition 
was the dominant process there, so far as present data show. 

Igneous activity during the Miocene. The widespread igneous 
activity which began with the close of the Cretaceous, perhaps 
reached its climax during the Miocene. Igneous materials abound 
in the sedimentary formations of the 


system throughout the west, and igneous | 


activity affected nearly or quite every state isa 
urg 
west of the Rocky Mountains, the erup- Formation 
: 1000-1500 ft. 
tions being from fissures as well as 


volcanoes. Among the conspicuous centers 
of activity the basin of the Columbia | 
and the Yellowstone National Park may | 
be mentioned. Locally, forests were Yakima | 
buried by the volcanic ejecta, and in 1000-2000 f,” 
favorable situations their trunks were 
petrified (Fig. 495). The lavas of at least 
a considerable part of 200,000 or 300,000 Tanewm 

; | Andesite 
square miles of lava-covered country in 


| 
the western part of the United States = | Manatash : 
S Formation 


MIOCENE 


issued during this period, or during the 
time of crustal deformation which brought 
it to a close. Volcanoes were active in 
the Antillean region of Central America ae AYR 
and the West Indies, and the Andean | 
system of South America, as well as in Stes 
d Fig. 493. Columnar sec- 

North America. tion showing the succession 

Close of the Miocene. Slow warpings of formations in central 
of the surface seem to have been in prog- U.S Gea Sure) Smith, 
ress throughout the Cordilleran region 
during the Miocene period, accompanied by faulting and vulcanism, 
and locally, by pronounced orogenic movements; but toward the 
close of the period movements were more general. Pronounced 
deformation affected the coastal regions of Oregon and northern 
California, tilting and folding the Miocene and older formations. 
The principal folding of the existing Coast Ranges of both these 
states has been assigned to this time, but it now appears that some 
of the deformations formerly referred to the end of the Miocene took 





PRE-TERTIARY 


588 THE MIOCENE PERIOD 


place earlier (p. 583). The Cascade Mountains of Washington were 
in process of growth at this time.! ! 

Similar movements were widespread east of the coast, as in the 
Great Basin region and elsewhere. In some places, they deformed 
strata heretofore horizontal, but more commonly they affected 
formations and areas which had suffered deformation earlier. 

The later part of the period was perhaps the time when the 
greater relief features of the rugged west, as they now exist, were 





Fig. 494. Courthouse and Jail Rocks. Buttes of the Arikaree soca) 
formation of western Nebraska. (Darton, U.S. Geol. Surv.) 


initiated. The great relief features of earlier times appear to have 
lost their greatness before this time. After the movements of the 
late Miocene had been accomplished, it is probable that the western 
part of the continent had a topography comparable, in its relief, to 
that of the present, though by no means in close correspondence with 
it. The details, and many of the larger features, of the present 
topography are of still later origin. 

In the eastern part of the continent, the geographic changes 
were less, though the Atlantic and Gulf regions seem to have 
emerged, shifting the coast-line to some such position as it has 
today. 


Foreign 


Europe. As compared with the Eocene, the sea on this conti- 
nent was somewhat restricted in the north, and somewhat extended 


1 Willis, Professional Paper 19, U. S. Geol, Surv. 


FORMATIONS AND PHYSICAL HISTORY 589 


in the south. As in most other post-Paleozoic systems, non-marine 
formations have much representation in this. The marine beds are 





Fig. 495. Petrified tree-trunks, Yellowstone National Park. (Iddings, U. S. 
Geol. Surv.) 


chiefly along the Atlantic and Mediterranean. In the north, much 
of the system is buried beneath glacial drift. Thick conglomerates 
(3,900-5,900 feet) of early and middle Miocene age are found 


590 _ THE MIOCENE PERIOD 


along the north base of the Alps, and tell something of the relief of 
the Alpine region at the time. Southern Europe appears to have 
been an extensive archipelago, the plateau of Spain, parts of the 
Pyrenees, the Alps, and the Carpathian Mountains, and portions of 
adjacent lands being islands. The sea of southern Europe extended 
east far beyond the limits of the present Mediterranean, but late in 
the period it was much restricted. 

The Miocene formations include all the common sorts of sedi- 
mentary rocks common to marine and non-marine deposits. The — 
latter include not a little limestone of fresh-water origin, made 
partly from the secretions of alge. In Italy the system is said to 
have a thickness of nearly 6,000 feet. 

Considerable disturbances occurred in the later part of the 
period, and at its close. Before its end, the Alps had had a period 
of growth, usually placed at the close of the Lower Miocene. The 
Apennines and other mountains of southern Europe also were in 
process of development during the later Miocene. In the Caucasus 
Mountains, Miocene beds occur up to heights of 2,o00 meters. As 
in America, widespread movements which were not notably 
deformative attended the growth of the mountains, with the result 
that the sea which had overspread southern Europe was greatly 
restricted, though not reduced to its present size. Igneous activity 
appears to have attended the movements, but not on such a scale 
as in North America. 

Other continents. The Miocene of Asza has not been generally 
separated from the other Tertiary formations, but is known to be 
widely distributed in the southern part of the continent. In 
Africa, Miocene formations occur in Algeria and Lower Egypt, and 
are well represented in Australia and New Zealand. The beds are 
found up to heights of 4,000 feet, giving some clue to the extent of 
post-Miocene crustal deformation here. 

In South America, Miocene beds probably occur on the western 
coast, and are known to have extensive development on the eastern 
plains of the southern part of the continent,! where the distinction 
between Upper Oligocene and Miocene is not sharp. The lower part 
of the Oligocene-Miocene series (Patagonian beds) is marine, while 
the upper part (Santa Cruz) is of fresh-water origin. The terrestrial 

1 Hatcher, Sedimentary Rocks of Southern Patagonia, Am. Jour. of Science, — 


Vol. IX, 1900; and Ortmann, Princeton Univ. Repts. of Expedition to Patagonia, 
Vol, IV, Pt. II, 


LIFE 591 


faunas of this region are strikingly similar to the Miocene and later 
faunas of Australia and New Zealand. 

Arctic latitudes and climate. Miocene beds are somewhat 
widely distributed in the Arctic regions and seem to be largely of 
terrestrial origin, with fossil floras indicating a warm temperate 
climate. 


LIFE 


Land Plants 


The mid-latitude flora of the Miocene records the gradual dis- 
appearance of subtropical types, and an increase of deciduous trees. 
This is particularly true of North America, where the flora came to 
resemble that of to-day in somewhat lower latitudes, and is indeed 
its predecessor. An important feature in North America was an 
increase in the grasses, with its appropriate effect on mammals. 


Land Animals 


Earlier fauna. The early Miocene land fauna of-North America 
was very distinct from the late Miocene. The former resembled 





Fig. 496. A Miocene Mastodon, Tetrabelodon angustidens Cuvier. (Restora- 
tion by Gaudry.) 


the Oligocene (White River) fauna. True carnivores, chiefly of the 
cat and dog families, had succeeded the primitive forms. Several 
branches of the perissodactyls had disappeared, reducing them 


592 THE MIOCENE PERIOD 


essentially to their three persistent lines, exemplified by the horse, 
the tapir, and the lowland rhinoceros. The even-toed branch also 
had developed into modern lines. Rodents were abundant, includ- 
ing squirrels, beavers, gophers, rabbits, etc. 

Later fauna. Elephants. A notable addition to the mam- 
malian fauna of North America in the late Miocene, was the probos- 
cidians. Primitive proboscidians lived in Egypt at least as early 
as the Middle Eocene, and in Europe in the early Miocene. Ele- 
phants reached North America in the late Miocene, and South 
America in the Pliocene. 

Much more important was the immigration of the modern 
ruminants. The great ruminant group that later formed so im- 
portant a part of the fauna does not seem to have descended from 
early North American forms, but to have come in from Eurasia. 
Their remains are found in the Loup Fork beds. The first immi- 
grants belonged to the deer and ox families. The earliest known 
deer (excluding Protoceras) were in Europe. They were hornless, 
as are their surviving relatives in Asia. By the middle of the 
Miocene, some of the males had acquired small two-pronged decidu- 
ous antlers. It was at this stage that they appeared in America. 
About the close of the period, three or four prongs were added, 
and in the Pliocene the antlers were variously branched. The 
Miocene skeletons imply lightness and speed, but not to the same 
degree as now. 

There is some doubt as to the precise stage to which the remains 
of bison found in Nebraska and Kansas are to be assigned. They 
usually have been referred to the Lower Pliocene; but Matthew 
assigns them to the Upper Miocene, and Williston to the early 
Pleistocene. The earliest known bisons on the Eurasian conti- 
nent were found in the Siwalik Lower Pliocene formation of India. 

The earlier genera of camels were gone, but 15 species of more 
modern type have been identified from the Loup Fork formation. 
The family seems to have been confined still to North America. 

Evolution of the horse. The Miocene was a great epoch in the 
evolution of the horse; Anchippus, Protohippus, Pliohippus (Mery- 
chippus), Hipparion, and other genera flourished, and forty or more 
species. They were still three-toed, but the two lateral toes were 
dwarfed and did not usually touch the ground, while the central one 
was strengthened and bore all the weight. A large group of struc- 
tural features were being modified concurrently with the feet, to fit 


LIFE | 503 


and Teeth like those of Monkeys etc. 


Three Toes 
Side toes 
not touching the ground 
Three Toes 
Side toes 
touching the ground; 
Splint of 5" digit 
Four Toes 
Four Toes 
Splint of 1* digit 


[Fore Foot [ Wind Foot [Teeth 
i Splints of ; ' 

haigi 2% ond 4' digits 

Three Toes 
Side toes 

Three Toes 
Side toes 

touching the ground 

Three Toes 

Splint of Stdigit 

Hypothetical Ancestors with Five Toes on Each Foot 


Hyracotherium 


(After William D. Matthews, Am. Mus. Jour.) 


9 Mesohippus 


ae Protorohippus 






































Kf 

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I } tl Wiley 
lycitia ih 

i ty inv 

sea 


(4) 
<2) 
oa 
>) 
am) 
ca 
ae 
be 
fu. 
oO 
Zz 
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E~ 


The evolution of the horse. 






























































































































































































































































Fig. 497. 





oar ae fore Formations in Western United States and Characteristic Type of Horse in Each 


504 THE MIOCENE PERIOD 


the evolving horse to dry plains and grassy food (Fig. 497). The 
elimination of the side toes, the lengthening of the limbs, the con- 
centration of the limb muscles near the body to reduce the weight 
of the parts most moved, and the consolidation of the leg bones, were 
modifications in the interest of speed and strength. An elongation 
of head and neck was necessary to reach the ground. The front 
teeth were reduced to chisel-like, cropping forms, while the molars, by 
developing ridges, became suited to grinding. The teeth also grew 
in length to provide for the great wear caused by the dry siliceous 
grasses. It is probably as safe to infer a development of dry, grassy 
plains from this evolution of the horse as to infer climatic and 
topographic conditions from plants and other organic adaptations. 

Other orders. Tapirs were but meagerly represented, but 
rhinoceroses were prominent. Most of the American species were 
hornless, but two-horned species appeared during the period in 
Europe. Carnivores were abundant, and had assumed forms re- 
ferred with some doubt to the living genera. The dog family in- 
cluded numerous wolves and foxes; the cat family, panther-like . 
animals and saber-toothed cats; weasel-like and otter-like forms, 
and an ancestral raccoon represented another family. The genera of 
the late Miocene were nearly all different from those of the early 
Miocene, indicating rapid evolution. Rodents were abundant, but 
neither insectivores nor primates are among the North American 
fossils. ‘The development of the plains, which favored horses, deer, 
and cattle, was obviously unfavorable to the lemuroids. 

Primates. In the Old World, apes had appeared. One type was 
rather large, combining some of the characters of apes and monkeys; 
another was related to the chimpanzee and gorilla, and about as 
large as the former. It is the view of some paleontologists that the 
ancestral branch of the Hominide (man) must have diverged from 
its relatives at least as early as this; but on the origin of man the 
geologic record throws no direct light. 

Lower vertebrates. Little of moment is recorded rele to the 
lower vertebrates. Not much is known of American Miocene birds, 
but their advancement in later stages implies that they continued 
their evolution with measurable rapidity, a conclusion supported 
by the European evidence. Reptiles were represented by turtles, 
snakes, and crocodiles. Amphibians came again to notice in the 

! For a recent illustrated statement of the evolution of the horse, see Matthew, 
Supplement to Am. Mus. Jour., Vol. III. 


LIFE 595 


form of a large salamander, whose remains, found at Oeningen, 
Switzerland, formerly attained an unworthy celebrity from false 
identification as a human skeleton, and from the application of the 
pretentious name Homo diluvit testis. 

Summary. A general view of the American Miocene land fauna 
shows that the great order of ungulates took precedence in evolu- 
tion, and that both the odd- and even-toed branches participated 
actively. Closely following these in importance, and dependent 
on them for the conditions of their evolution, came the carnivores. 
Rodents occupied a middle position, and insectivores and lemuroids 
declined notably. 

The European record bears a similar general interpretation, 
with the ungulates somewhat less pronouncedly in the lead, the 
carnivores somewhat better deployed, and the proboscidians a 
conspicuous factor. The important evolution of the higher pri- 
mates seems to have been confined to the Old World. 


Marine Life 

Provincialism dominant. The pronounced provincialism that 
had been inaugurated in the Oligocene epoch continued throughout 
the remainder of the Cenozoic era, being favored by the shallow 
seas about North America, and the bays and straits of Europe. 
Even the narrow border tracts that were geographically continuous 
show signs of having been cut into biological sections by interrupting 
barriers. The land being extensive, large rivers reached the coast 
here and there, and poured great volumes of fresh and muddy waters 
across the shore belt, doubtless forming barriers to some species. 
The warpings of the crust probably developed submarine ridges on 
the continental shelf. These were not only barriers in themselves, 
but had an influence in directing the courses of the coast currents. 
Differences of climate in different latitudes had been developed, 
apparently, and cold and warm currents were probably more pro- 
nounced than in earlier times, and their shiftings had still graver 
effects upon the faunas. So, too, the lower temperatures in the 
northern shore tracts of the Atlantic and Pacific prevented their 
serving longer as migratory routes for warm-water species, and this 
tended further to intensify the provincial nature of the shallow- 
water faunas. 

According to Dall, the Chesapeake Miocene was ushered in by a 
marked faunal change due to a cold northern current driving out 


596 THE MIOCENE PERIOD 





Fig. 498. MrocENE PELECypops: a and b, Arca (Scapharca) staminea Say; 
c and d, Corbula idonea Conrad; e, Crassatellites marylandicus (Conrad); f, Phacoides 
(Pseudomiltha) foremani (Conrad); g, Tellina (Angulus) producta Conrad; h, Leda 
concentrica (Say); 1, Modiolus dalli Glenn; 7, Astarte thomasit Conrad; k, Ensis 
directus (Conrad); 1, Spisula (Hemimactra) marylandica Dall; m, Isocardia markoéi 
Conrad; », Cardium (Cerastoderma) leptopleurum Conrad; 0, Pecten (Chlamys) 
madisonius Say; p, Venus ducatelli Conrad; q, Ostrea carolinensis Conrad. (Mary- 
land Geol. Surv.) | 





Fig. 499. M1ocENE Gastropops (one Scaphopod): a, Turritella variabilis 
Conrad; b, Scala sayana Da!l; c, Nassa marylandica Martin; d, Terebra unilineata 
Conrad; e, Solarium trilineatum Conrad; f, Cancellaria alternata Conrad; g, Surcula 
biscatenaria Conrad; h. Calliostoma philanthropus (Conrad); 7, Acteon shilohensis 
Whitfield; 7, Oliva litterata Lamarck; k, Retusa (Cylichnina) conulus (Deshayes); 1, 
Conus diluvianus Green; m, Polynices (Neverita) duplicatus (Say); n, Fissuridea 

Continued on next page. 


508 THE MIOCENE PERIOD 


or destroying the previous warm-water fauna of the region, and 
bringing with it a cold-water fauna. There was a complete change 
of species, and even some genera were displaced. The fauna re- 
tained, however, a general molluscan aspect. Both the bivalves 
and the univalves gave proof of better adaptability to the vicissi- 
tudes of the coastal tracts than most other forms, and held their 
dominance. Figs. 498 and 499 show a few characteristic types. 
Compared with the Eocene group, Fig. 482, the resemblances will 
be found more striking than the differences. 

The marine fauna of the Pacific coast indicates a climate but 
little warmer than that of the present, and this conclusion is re- 
enforced by the plants of the Puget-Sound region, which record 
a transition from the subtropical climate of the Eocene to the tem- 
perate climate of the present. The fauna of the Upper Miocene 
indicates a still closer approach to the present. 
alticosta (Conrad); 0, F. griscomi (Conrad); p, Xenophora conchyliophora (Born); 
q, Crepidula fornicata (Linné); r, Fulgar spiniger (Conrad) var.; s, Ecphora quadri- 


costata (Say); t, Siphonalia marylandica Martin; u, Ilyanassa (?) (Paranassa) 
porcina (Say). Scaphopod: v, Dentalium attenuatum Say. (Maryland Geol. Surv.) 


CHAPTER XXVIII 
THE PLIOCENE PERIOD 
FORMATIONS AND PHYSICAL HISTORY 


Subaérial Formations 


The most distinguishing feature of the Pliocene formations, so 
far as the present continents are concerned, is the predominance of 
terrestrial deposits. This is a consequence of (1) the exceptional 
deformations which took place during the period, and before its 
beginning, and (2) the recency of the period, which has saved its 
deposits, to a large extent, from removal. Similar deposits after 
earlier periods of comparable deformation have been largely removed 
by later erosion. These deposits of the Pliocene are perhaps 
most obvious in intermontane regions such as the Great Basin. 
They have by some been interpreted as lacustrine deposits, and 
such no doubt exist; but over areas much greater than those oc- 
cupied by Pliocene lakes, and over tracts which were never parts 
of well-defined flood plains, broad aprons of detritus accumulated. 
Most of the western mountains of America are flanked by such 
deposits of Pliocene age, or younger. Pliocene deposits of this type 
are doubtless concealed beneath later accumulations of a similar 
sort in nearly all the large basins, and at the bases of nearly all the 
steep slopes in the western mountain region. 

In the Mississippi basin, far from all mountains, there are 
patches of gravel on various hills and ridges which are interpreted 
as the remnants of a once more or less continuous mantle of river 
detritus. Definite correlation of these gravels is not now possible, 
and they may not all be of the same age. They are not older than 
Cretaceous, and are older than the glacial drift. Their similarity 
to the Pliocene gravels farther south suggests their correlation with 
that formation. The material of these gravels, almost wholly 
quartz, quartzite, and chert, is partly local, and partly from the 
north. The leading topographic features of the Mississippi basin 


599 


600 THE PLIOCENE PERIOD 


have been developed since their deposition, for their remnants are 
on the highest lands within the area where they occur. 

The Lafayette formation.! About the Atlantic and Gulf coasts 
similar deposition gave rise to the Lafayette (Orange Sand) forma- 
tion, which seems to have had a history somewhat like that of 
the Pliocene beds of the west, though this interpretation has been 
challenged. This formation has an extensive distribution (1) 
between the Piedmont plateau and the Atlantic, (2) on the inland 
part of the Coastal Plain of the Gulf of Mexico, and (3) in the south- 
ern part of the Mississippi basin, and is represented, if our inter- 
pretation is correct, (4) in some of the valleys of the Appalachians 
and west of them. On the Coastal Plain of Texas the formation 
is connected with analogous deposits on the Great Plains, and 
through them with the intermontane deposits of the west, already 
mentioned. The term Lafayette has been applied only to the 
formation on the slope between the Appalachians and the Atlantic, 
about the Gulf, and in the Mississippi basin below the Ohio, where 
it lies upon the eroded edges of older formations, and extends in- 
land from the coast up to altitudes of 1,000 feet? near the Rio 
Grande, 800 feet in Tennessee, and 300 to 500 feet on the Atlantic 
slope. At its mountainward edge, ragged belts of the Lafayette 
formation follow the valleys up into the mountains. At its seaward 
margin, it is more or less completely concealed by younger beds, and 
it is not to be doubted that it passes out to sea beneath them. No 
part of the formation on land is demonstrably marine. 

Within the general area of its distribution the formation is not 
continuous. Over considerable areas, it caps divides, but is absent 
from the valleys between them, obviously the result of stream ero- 
sion. The base on which the formation rests has but little relief, 
and a gentle dip seaward. 

In general, the formation thickens seaward. Its known thick- 
ness ranges from o to 200 feet or more, sections of 20 or 30 feet 
being common. 

1 The fullest sketch of this formation as a whole is that of McGee in the Twelfth 
Annual Report of the U.S. Geological Survey. A few references to other accounts 
of the formation in special localities, some of them under other names, are as follows: 
Safford, Am. Jour. Sci., Vol. XX XVII, 1864; Hilgard, Agric. and Geol. of Miss., 
1860, and Am. Jour. Sci., Vol. XLI, 1866, and Vol. IV, 1872; Salisbury, Geol. 
Surv. of Ark., Report on Crowley’s Ridge, 1889; Dumble, Jour. Geol., Vol. I, 


1894, p. 560; Smith, E. A., and Johnson, L. C., Geol. Surv. of Ala., 1894. 
2 McGee, loc. cit. 


FORMATIONS AND PHYSICAL HISTORY 6o1 





Fig. 500. Map showing the distribution of the better-known parts of the 
Pliocene system. The area of the Lafayette, along the Atlantic and Gulf coasts, 
is marked by vertical dashes. This formation doubtless is more widespread than 
the map shows. Relatively little of the exposed Pliocene is marine. 


602 THE PLIOCENE PERIOD 


It is composed of gravel (and occasionally bowlders), sand, 
silt, and clay, variously related to one another. It may be said to 
be both heterogeneous and homogeneous; that is, there is consider- 
able variation in composition in short distances, and but little more 
in great ones. In the lower Mississippi basin, whence the name is 
derived (Lafayette County, Miss.) it is of sand and gravel chiefly, 
having in many places the distinctive characteristics of fluvial sand 
and gravel. Over a broad tract of the uplands east of the Missis- 
sippi and away from valleys generally, it is composed largely of silt 
and clay. Its constituents are chiefly the insoluble residues of 
older formations farther up the continental slope on which it lies, 
chert and quartz pebbles making up its gravels, and other insoluble 
matter its fine constituents. These constituents replace one 
another at short intervals and in various ways, and no systematic 
succession is observable. Irregular stratification is the rule, but 
some portions are not bedded. Certain lenses of sand suggest an 
eolian origin, and pebbly-earths that find their analogue in subaérial 
and flood-plain deposits are common. The color of the formation 
ranges from brick-red through various pinks, purples, oranges, and 
yellows, to white. The color is more irregular than the composition, 
bands, blotches, and mottlings diversifying the structural units. 
Fossils are rare. In its representative parts they are all of land 
plants and animals (except, of course, the fossils derived from earlier 
formations). 

Origin. ‘The preferred interpretation of the Lafayette formation 
is as follows: At the opening of the Pliocene, the Appalachian tract 
is supposed to have been affected by broad, flat, intermontane val- 
leys, mantled by a deep residual soil and subsoil. The Piedmont 
tract to the east is supposed to have been a peneplain near sea-level. 
It is assumed that the upward bowing was felt first in a relatively 
narrow belt along the axis of the mountain system, that the rise 
was gradual, and that the rising arch increased in width as time 
advanced. The first up-bowing rejuvenated the head waters of 
the streams from the mountain tract, and the surface, with its 
heavy mantle of residual earth, readily furnished load to the streams. 
When they reached that portion of the peneplain not yet affected, 
or less affected, by the bowing, they dropped part of their load 
(at b, Fig. 501). With continued rise, the zone of deposition is sup- 
posed to have been shifted seaward, and the deposits already made 
were eroded and the eroded material was redeposited farther from 


FORMATIONS AND PHYSICAL HISTORY 603 


the mountains and nearer the sea (at b’, Fig. sor). Thus the process 
is presumed to have continued till the border of the upraised tract 
passed beyond the present sea-coast. The whole deposit within the 
area of the present land was then eroded, and the erosion had 
reached a notable degree of advancement before the first known 
glacio-fluvial deposits were laid down. This hypothesis of the origin 
of the formation postulates that the shallow valleys of the coastal 
plain were filled with sediment, and that later the deposits spread 
rather generally over the low divides between them. In the region 
of deeper valleys, such as the Tennessee, the valleys were only partly 
filled. It has been assumed generally that the formation was once 


$< << 
a 


Fig. 5or. Illustrating the progressive stages of arching described in the text, 
and the attendant shifting zones of deposition; s-s, sea-level; a, original peneplaned 
surface with graded slope to sea-coast; a’, a’’, a’’’, successive stages of arching; 


, b’, b’’, b’”’, successive sites of deposition corresponding to stages of arching a, a’, 
a”’,a’"’. In the stage of arching represented by a’, the right hand portion of the 


previous site of deposition is lifted and becomes a part of the area of erosion. The 
same process is carried farther in the next stage represented by a’”. 


continuous east of the mountains where patches only now remain; 
but it may be that the higher divides were never covered by the 
formation. 

The removal and re-deposition of material as suggested by Fig. 
501 is regarded as an important part of the interpretation of the 
formation. Erosion and re-deposition of the material did not cease 
with the Lafayette epoch, but have been in progress ever since, and 
the derivatives so closely resemble the parent formation in structure 
and material that their separation is difficult. 

If it shall be shown ultimately that the seaward portions of the 
Lafayette, now concealed or unstudied, are marine, the preceding 
hypothesis would need to be modified only by supposing that as 
the sources of the streams was bowed up, the coastal border of the 
plain was submerged. In this case, there should have been estuarine 
formations in the seaward ends of the valleys. 

The chief alternative view relative to the origin of this forma- 
tion regards it as marine,' deposited during a stage of submergence 
essentially co-extensive with the area of the formation. This 
hypothesis has been tried out by geologists of wide familiarity with 


1 McGee, 12th Ann. Rept., U. S. Geol. Surv, 


604 THE PLIOCENE PERIOD 


the phenomena, and abandoned as untenable even where conditions 
seem most to favorit. The objections to it are (1) the absence of 
marine fossils; (2) the presence of structural features not indicative of 
typical marine deposits; (3) the chemical condition of the formation, 
particularly the high and very unequal oxidation and the meager 
hydration; (4) the topographic relations of the formation, especially 
the lack of any approach to horizontality in its upper limit; and (5) 
the absence of shore phenomena. 


Marine Formations 


The Atlantic coast. If fossils be the test, Pliocene beds of 
marine origin have little development on the eastern side of the 
continent. In Florida only (Caloosahatchie beds) have beds con- 
taining marine fossils any considerable extent at the surface, though 
small patches are known farther north. They may be parts of a 
continuous formation, chiefly concealed. The time relations of 
these marine Pliocene beds to the Lafayette are undetermined. 
Pliocene beds of marine origin have not been identified certainly 
between Florida and Texas, but they cover considerable areas farther 
south, as in Yucatan. 

The Pacific coast... Marine sedimentation along this coast was 
confined to narrow limits (Fig. 502). The deposits are chiefly 
clastic. Their maximum known thickness is found south of San 
Francisco, where about 4,000 feet of strata (Merced series) are 
exposed.” The non-marine part of the system (Paso Robles forma- 
tion) is as thick in the San Joaquin valley. 


Crustal Movements * 


The tendency to crustal movements, both warping and faulting, 
which had characterized the western part of the continent since 
thé close of the Mesozoic, seems to have continued at least inter- 
mittently through the Pliocene. Perhaps these movements were 
in many places no more than continuations of those begun earlier. 

About the close of the period, movements were extensive and 

1 Arnold, Ralph, Jour. Geol., Vol. XVII. 

2 Lawson, Science, Vol. XV, 1902, p. 410, and Hershey, Am. Geol., XXIX, 
p. 359, give the Pliocene of California greater thicknesses. 

3 LeConte, Am. Jour. Sci., Vol. XXXII, p. 167, 1886, Bull. Geol. Soc. Am., 
Vol. II, p. 329, Jour. Geol., Vol. VII, p. 546, 1899; Hershey, Science, Vol. III, 
p. 629, 1896, and Dutton, Mono, I, U, S, Geol, Surv. 


FORMATIONS AND PHYSICAL HISTORY 605 


great, resulting in increased height of land. The region covered by 
the Lafayette formation was elevated relatively, and perhaps some- 
what deformed. ‘The coast line was probably farther east than now, 
perhaps at the edge of the con- 
tinental shelf. To this epoch the SPSS 
submerged continuations of the SS eth weve 

St. Lawrence, Hudson, Delaware, (OU ree suwarer C= 
Susquehanna, and Mississippi 
valleys are commonly referred. 
From these submerged valleys it. 
was formerly assumed that the 
land along the Atlantic seaboard 
must have stood 2,000 to 3,000 
feet, or perhaps even 7,000 to 
12,000 feet! above its present 
level, to allow of their excavation; 
but it may not be necessary 
to postulate such extraordinary 
changes of level. Continental 
creep (p. 350) along the slope be- 
tween the continental platforms 
and the ocean basins may have 
lowered the valleys notably as it 
carried them seaward, if such creep 
is a fact. 











“i 


==V 





| (ei 
Aas! 















































In the Mississippi basin also == 
there was notable elevation at the ——— 
close of the period, though prob- a 
ably less than has sometimes been |.) Priocene. 
estimated. It seems possible, or Raph Amaaraca F 





‘perhaps even probable, that the Fig. 502. Map showing supposed 

evolution of the principal physio- distribution of land and water on the 

graphic features of the interior, so Pacific coast of the United States 
; ; during the Pliocene period. (Ralph 

far as due to erosion, is post- Arnold.) 

Pliocene. 

In the west, too, there were notable closing-Tertiary movements. 
The plateau region was in process of uplift, periodically, through- 
out the Tertiary, during which it has been estimated to have under- 
gone an elevation of 20,000 feet (Dutton), and a degradation of 


1 Spencer, Am. Jour, Sci., Vol. XIX, 1905. 


606 THE PLIOCENE PERIOD 


12,000, leaving it 8,000 feet above sea-level. How much of this is 
assignable to the close of the Pliocene is uncertain. It was Dutton’s 
view that the Colorado plateau was so elevated at this time as: to 
rejuvenate the Colorado River, and that the cutting of its inner 
gorge some 3,000 feet (maximum) below the outer (Fig. 73), was the 
work of later times. More recent studies indicate that even the 
outer and broader part of the valley is younger than formerly was 
thought, and raise a question as to whether the inner gorge is not the 
result of rock structure, rather than of a distinct and later uplift. 
If the whole of the canyon is post-Pliocene, the elevation of the 
region since the close of the Tertiary must have been several thou- 
sand feet. The later elevations in this region, largely by blocks, 
were so recent that many of the fault scarps are distinct, and in- 
dependent of stratigraphy and drainage. 

In the basin region, faulting and deformation ” gave rise to de- 
pressions between the Sierra Nevada and the Wasatch Mountains, 
preparing the way for two great Pleistocene lakes (Bonneville and 
Lahontan). It is probable that many other faults between the 
Rockies and Sierras were developed at the same time, and in many 
cases the movement seems to have been along fault planes estab- 
lished earlier. 

In the Sierra region, the post-Tertiary (or late Tertiary?) up- 
lift was still more marked.* Not only the deep canyons of these 
mountains, but all the scenery of the high Sierras is post-Tertiary.* 

Still nearer the Pacific, notable changes marked the transition 
to the Pleistocene. In some parts of southern California (Los 
Angeles County) marine Pliocene beds are said to occur up to alti- 
tudes of 6,000 feet, and in others (San Luis Obispo), there was fold- 
ing (Fig. 492) and faulting, while the shore-line was pushed out 
toward the edge of the continental shelf. There are submerged 
valleys along the Pacific coast, as along the Atlantic, but their 
excavation has been referred to a time earlier than the close of the 
Tertiary. 

In Washington, present knowledge points to the early Pliocene 
as a time of prolonged erosion. The crests of the Cascade Moun- 


! Huntington and Goldthwaite, Bull. Mus. Comp. Zodél. Geol. Ser., Vol. VI, 
p. 252; and Davis, ibid., Vol. XX XVIII. 

2 King, U. S. Geol. Expl. of the 4oth Parallel, Vol. I, p. 542. 

3 LeConte, op. cit., and Diller, 14th Ann. Rept., U. = Geol. Surv. 

4 The beginning of the re- -elevation of the Sierras, after peneplanation, was 
mid-Miocene, 


FORMATIONS AND PHYSICAL HISTORY 607 


tains seem to represent remnants of a deformed peneplain, which, 
carried to the east and south, is continuous with an erosion plain 
which cuts across strata (Ellensburg formation) of late Miocene ! 
age. The planation must, therefore, have been later than that 
part of the Miocene period represented by the Ellensburg formation. 
At least the early part of the Pliocene period, if not most of it, would 
seem to have been necessary for the accomplishment of this great 
planation, so that the peneplain can hardly be thought to antedate 
late Pliocene time. If this is correct, the main features of the present 
topography of this rugged region are the result primarily of Pleisto- 
cene erosion on the peneplain uplifted and deformed in Pliocene 
time, or later, and secondarily of vulcanism, which has built up 
the great volcanic piles (Rainier and others) which affect the region. 
In British Columbia also, the Pliocene is thought to have been pri- 
marily a time of erosion. 

Deformative movements of the orogenic type seem not to have 
been common at the close of the Pliocene, but such movements 
affected the Santa Cruz Mountains of California, where Miocene 
(Monterey) and Pliocene (Merced) beds were deformed together.” 

On the whole, the close of the Pliocene must be looked upon as 
a time of great deformation, a critical period in the history of North 
America. New lands were made by emergence from the sea, and 
old lands were deformed and made higher; new mountains were 
made, and old ones rejuvenated; streams were turned from their 
courses in some places, and nearly everywhere started on careers 
of increased activity. The fact that such notable changes, with 
increased elevation of land, occurred during the epoch next pre- 
ceding the glacial period, is one of the considerations which led 
to the once widespread belief that elevation was the cause of the 
climate of the latter period. While there may be a connection 
between the two things, it was probably not in the simple and com- 
monly accepted sense. 


Volcanic Activity 


The volcanic activity of preceding periods continued into the 
Pliocene, and became somewhat pronounced near the end of the 
period in different parts of the western Cordillera. Some of the 


1 Smith, Ellensburg, Wash., folio, U. S. Geol. Surv.; and Willis and Smith, 
Professional Paper 19, U. S. Geol. Surv. 
* Ashley, Jour. Geol., Vol. IIT, p. 434. 


608 THE PLIOCENE PERIOD 


late igneous formations of the Sierras, and perhaps of northern 
California,! belong to this time, and probably some of those of 
nearly or quite every other state west of the Rocky Mountains. 
Many of the prominent volcanic peaks of the west date from this 
time or later, and represent the later phases of the prolonged period 
of volcanic activity, just as the great lava flows and intrusions 
represent the earlier. Many lesser cones belong to the same period. 


Foreign 


From considerable areas of Europe covered by water during the 
Miocene, the waters retreated late in the period or at its close; but 
the sea covered southern and southeastern England, Belgium, and 
parts of France during at least some portion of the Pliocene, and 
still more extensive areas of the present continent about the Medi- 
terranean. Beyond the inland margins of the marine Pliocene, there 
are contemporaneous beds of terrestrial origin. In southeastern 
Europe, brackish and salt lakes came into existence, as shown by 
the fossils and the local deposits of salt and gypsum. In some 
places, as in the Vienna basin, brackish water beds below grade up 
into fluviatile beds above. 

In Italy only do Pliocene beds attain massive development. 
Along the Apennines their thickness has been estimated at from 
1,600 to 3,000 feet, and in Sicily 2,000 feet. Limestone as well as 
clastic beds enter into the system, which occurs up to heights of 
3,000 feet. 

Marine Pliocene is known in Egypt, where the sea is thought to 
have extended up the Nile to Assuan. The formation of the basins 
of the Red Sea and the Gulf of Suez has been assigned to this 
period. These depressions have been thought to be down-faulted 
blocks. 

LIFE 

Land plants. During the Pliocene there was a further sort- 
ing out of the mixed flora of previous periods, and the southerly 
segregation of what are now tropical and subtropical plants contin- 
ued; but in Europe generally there was still much commingling of 
species now separated geographically. | 

Land animals. Three important features characterized the 
Pliocene history of mammals: (1) A notable intermigration between 
the continents, including North and South America; (2) the begin- 

1 Hershey, Jour. Geol., Vol. X, pp. 377-392, 


LIFE 609 


ning of the present divergence between Old and New World types; 
and (3) the culmination and perhaps initial decline of the mammals, 
except those domestic species protected by man. 

The intermigrations of the early part of the period were made 
possible by the land connections brought about by deformative 
movements. The extent of the connection of North America with 
Asia at the northwest and with Europe at the northeast respectively, 
is uncertain, but there is conclusive evidence that there were good 
migratory routes for land mammals in both directions during a part 
of the period. There are also strong hints that the connection 
afforded passage for some species, but not for others, due perhaps to 
the increasing cold toward the end of the period. This low tempera- 
ture, with its effect on intermigration, was perhaps the chief factor 
in developing the difference between the inammals of the Old World 
and the New. 

The connection between North and South America introduced 
a biological movement of much interest. There appears to have 
been no effective isthmian thoroughfare for land animals between 
the earliest Eocene and the Pliocene. During the Eocene con- 
nection, a few North American mammals seem to have sent repre- 
- sentatives into South America, and these had evolved there on 
distinctive lines in the interval. A remarkable group of sloths, 
armadillos, and ant-eaters had developed from an edentate stem; 
strange hoofed animals of orders unknown elsewhere had arisen from 
some very primitive ungulate form; and the monkeys of the South 
American type had evolved probably from a North American Eocene 
lemuroid. That the connection of the continents in the Eocene 
was only partial or temporary seems to be implied by the absence 
in South America of most of the great North American groups. 
The absence of proboscidians in South America implies lack of con- 
nection between that continent and Africa, where these forms de- 
veloped during the Eocene and Miocene; but the many marsupials 
of South America, similar to those of Australia, imply either land 
connection between those continents, or striking parallel evolution. 
The South American mammalian fauna at the beginning of the 
Pliocene is a striking instance of evolution on a large scale in com- 
parative isolation, and in relative freedom from the severe stimulus 
of effective competition, powerful carnivores, and shifting geo- 
graphic relations.' 

1 Reports of the Princeton University expedition to Patagonia, 1896-99 


610 THE PLIOCENE PERIOD 


When connection between the two Americas was made in the 
Pliocene, the fauna of each continent invaded the other. Horses, 
mastodons, deer, carnivores, and tapirs from the northern continent 
went to the southern, while gigantic sloths from the south came to 
our continent, though they did not maintain themselves long. 

The herbivores had the foremost place among mammals; both the 
odd- and even-toed ungulates evolved their present orders, and many 
of their present genera. They were represented also by many 
genera and species which are now extinct. The evolution of the - 
horse was advanced to the existing genus Equus. Giraffes and 
giraffe-like animals, some of them of great size, invaded southern 
Europe and Asia, probably from Africa. 

The giants of the period were the proboscidians. ‘The extinct 
Dinotherium was widely distributed in Europe and has been found 
in India, but is not known to have reached America. Mastodons 
seem to have lived in all the continents, but it is doubtful whether 
elephants reached America before the Pleistocene. They appear 
to have flourished in Europe, and, with the associated rhinoceroses 
and hippopotamuses, gave the European Pliocene fauna an African 
aspect. 

Carnivores throve and perhaps gained on the herbivores; at any - 
rate they put a severe tax on the herbivores, forcing further progress 
in the line of alertness, sagacity, speed, and defense, and gaining 
similar qualities themselves. 

Great interest attaches to the development of the primates 
(monkeys, apes, man), but the data on this point are likely to re- 
main limited until the tropical regions of the Old World, where the 
chief evolution of this group seems to have taken place, are more 
fully studied. No remains of lemuroids or of their descendants 
have been found in the Pliocene of North America, and those of 
Europe are from the middle and southern parts of the continent, per- 
haps implying that northern Europe was too cold for these animals. 

Some years ago a man-like skeleton was found in what were then 
regarded as Pliocene deposits in Java, and named Pithecanthropus 
erectus. ‘The find included the roof of a skull, two molar teeth, and 
afemur. The form of the femur indicates that its possessor walked 
erect. The forehead was low and the frontal ridge prominent, and 
in general the characteristic features were intermediate between 
those of the lowest men and the highest apes, as shown in Fig. 503. 
The size of the brain was about two-thirds that of an average man. 


LIFE 611 


The interpretation of this find has elicited much difference of opin- 
ion. By some the bones are thought to be those of an abnormal 
man; by others, those of an ancestral type between man and his 
remote ancestry. Recent studies throw doubt on the Pliocene 





Fig. 503. Profile of the skull of the Pithecanthropus erectus (line Pe) compared 
with profiles of the lowest men and highest apes; Spy J and Spy JJ, the men of Spy; 
Nt, the Neanderthal man; H1/, a gibbon (Hylobates leuciscus); Sm, an Indian ape 
(Semnopithecus maurus); and At, a chimpanzee (Anthropopithecus troglodytes). 
(After Marsh.) 


age of the beds in which the fossil was found.t They may be 
Pleistocene. 

Marine life. The record of marine life on the Atlantic coast of 
America is meager, but it appears that species which then ranged 
from Bering Sea to the north Atlantic are now confined to temperate 
latitudes.?, On the coast of California the early Pliocene faunas 
indicate a temperature lower than that of the Miocene, while the 
later Pliocene faunas point to sub-boreal conditions.* On the other 
hand, Pliocene fossils from Alaska (vicinity of Nome) indicate for 
this locality a climate similar to that of north Japan and the 

1 Berry, Science, Vol. XXXVII, p. 418. 


2 Dall, Jour. Geol., Vol. XVII. 
3 Arnold, Ralph, Jour. Geol., Vol. XVII. 


612 THE PLIOCENE PERIOD 


Aleutian Islands, where the sea remains unfrozen. Pliocene fossils 
from the northwest coast of Iceland indicate a temperature no 
colder than 42° (mean), where conditions are now arctic. The 
apparent lack of harmony between the phenomena of California 
and higher latitudes may perhaps be due to the different horizons 
from which the fossils come, the fossils from the different places 
recording the climate of different parts of the period. 

Certain fossils of Japan and California indicate intermigration, 
or migration from a common center, some time during the period. 


CHAPTER XXIX 
THE PLEISTOCENE OR GLACIAL PERIOD 
FORMATIONS AND PHYSICAL HISTORY 


The distinguishing feature of this period is its extensive glacia- 
tion. Thick sheets of ice, having the slow movement of glaciers, 
covered six or eight million square miles of the earth’s surface where 
climates had been mild not long before. 

More than half the area known to have been glaciated during 
this period was in North America, and more than half of the re- 
mainder in Europe. 

North America. Nearly half of North America was covered by 
ice (Fig. 504), and strangely enough it was the plain, rather than 
the mountainous part, which had most ice. Three principal centers 
whence ice moved have been recognized on the continent,+ the 
Labradorean, the Keewatin, and the Cordilleran. Spreading from 
these centers, ice-sheets covered some 4,000,000 square miles. 
From the Labradorean center, the extension was notably greatest 
to the southwest, and in this direction the limit is some 1,600 miles 
from the center of dispersion, in latitude about 37° 30’. The exten- 
sion of the Keewatin ice-sheet to the southward was scarcely less. 
It found its limit in Kansas and Missouri, about 1,500 miles from 
its center, while to the west and southwest it extended 800 to 1,000 
miles toward the Rocky Mountains. One of the notable features 
of the ice dispersion was the great extension of the Keewatin sheet 
westward and southwestward over what is now a semi-arid plain, 
rising in the direction toward which the ice moved, while glaciers 
from the mountains on the west pushed eastward but nee beyond 
the foothills. 

The Cordilleran ice-sheet is less simply defined. iach of it 
occupied a plateau hemmed in by mountains; but plateau glaciation 
was complicated by extensive mountain Biseition of alpine type. 


1A fourth center (Patrician) has been suggested by Tyrrell, southwest of 
Hudson Bay, and still another by Wilson, in the extreme East. Wilson, The 
Glacial History of Nantucket and Cape Cod. 


613 


614 THE PLEISTOCENE PERIOD 


The southerly lobes of the complex body of ice crossed the boundary 
of Canada into the United States. The plains of Alaska seem to 
have been largely free from glaciation even when the waters of the 


hy 
on) 


‘ yy 
yITHOY OY, yyy 


hy 


GUS g SES 


Y 
s c 
q 
9 


iX} “> 


(Ce 
ac 


( 


ye 
nee 
hd 
~ 
~, 
‘ 
‘ 





Fig. 504. Sketch-map showing the North American area covered by ice at the 
stage of maximum glaciation. 


Ohio and the Missouri, 2,000 miles farther south, were being turned 
from their courses by the ice-sheets. 

South of the more or less continuous Cordilleran glaciation of the 
north, local glaciers were widely distributed in the western moun- 
tains, even down to New Mexico, Arizona, and southern California. 


DISTRIBUTION OF ICE 615 


They were larger at the north and smaller at the south. Of gla- 
ciation in the mountains of Mexico little is known. 

Greenland was glaciated more extensively than now. Newfound- 
land seems to have had its own ice-sheet, and the same was probably 
true of Nova Scotia, and probably of the peninsula between the Bay 
of Fundy and the lower St. Lawrence. 

Other continents. South of the ice-sheet of Europe (Fig. 505), 
great glaciers descended from the Alps to the lowlands in all direc- 


Sia 





F ie. 505. een showing the area of Europe covered by the continental 
glacier at the time of its maximum development. (Jas. Geikie.) 


tions. Iceland was buried in ice, and even Corsica had glaciers. 
In Asia glaciers larger than those of to-day affected all the higher 
mountains, and ice-sheets existed in some of the more northern 
lands. In tropical regions, there were glaciers in mountains where 
none exist now, and in mountains where there are glaciers now, the 
ice descended to levels 5,000 feet or more below its present limits. 
The southern hemisphere was affected less than the northern, but 
the higher mountains generally bore glaciers, and even mountains 
which were not very high, as the southern Andes, had glaciers which 


616 THE PLEISTOCENE PERIOD 


reached the plains outside the mountains. Antarctica is assumed 
to have been buried beneath ice as now. 


The Criteria of Glaciation 


The area of North America which was overspread by ice is 
covered by a mantle of clay, sand, and bowlders which, taken to- 
gether, constitute the drift. The various lines of evidence which 
have led to the general acceptance of the glacial theory have to 
do with (1) the drift, (2) the surface of the rock which underlies 





Fig. 506. ‘‘Pilot Rock,” a glacial bowlder near Coulee City, Wash. (Garrey.) 


it, and (3) the relations of the drift to the bed. Some of the prin- 
cipal considerations are the following:! 

1. Constitution. One of the distinctive characteristics of the 
drift is its heterogeneity, both physical and lithological. It is made 
up at one extreme, of huge bowlders (Fig. 506), and at the other of 
fine earthy matter. Between these extremes there are materials of 
all sizes, and the proportions of coarse and fine are subject to great 
variations. Coarse materials are, on the whole, most abundant in 
regions of rough topography, where the underlying and neighboring 
formations in the direction from which the drift came are resistant; 

‘ Jour. Geol., Vol. II, pp. 708-724 and 807-83 5, and Vol. III, pp. 70-97. 


CRITERIA OF GLACIATION 617 


fine materials are most abundant where the underlying formations 
and neighboring formations in the direction from which the drift 
came, are weak. The fine part of the drift is made up largely of the 
same materials as the gravel and bowlders, but of these materials in 
a fine state of subdivision. The coarse and the fine materials are, 
as a rule, mixed without trace of assortment or arrangement. ‘The 
drift of any locality is likely to contain rock material from every 
formation over which the ice which reached that locality had passed; 
but the larger part of the drift of any place is from formations near 





ape 


Fig. 507. A large bowlder in northwestern Illinois. (Carman.) 


at hand. Over large areas it is probable that 75% of the drift 
was not moved 50 miles.' No agent except glacial ice makes de- 
posits with these characteristics. 

2. Bowlders of the drift. Many of the bowlders and smaller 
stones of unstratified drift have smooth surfaces, but they are not 
generally rounded. Many are subangular, and the wear which 
they have suffered was effected by planing and bruising, rather than 
by rolling (Figs. 147 and 508). Some of these planed, subangular 
bowlders and stones are distinctly marked with one or more series of 
lines or stri@ on one or more of their faces. The lines of each series 
are parallel, but those of different sets may cross at any angle. By 
no means all the stones of the drift show stria. They are rarely 
seen on those which have lain long at the surface, and they are more 
common on the less resistant sorts of rock, such as limestone. No 

1 The Local Origin of the Drift, Jour. Geol., Vol. VIII, p. 426. 


618 THE PLEISTOCENE PERIOD 


depositing agent except glaciers habitually marks the stones which 
it deposits in this way. 

3. Structure. The larger part of the drift is unstratified, but 
a considerable part is stratified, some of it irregularly. The un- 
stratified drift (Fig. 509) or dill (for some of it the name bowlder-clay 
is appropriate) has no orderly arrangement of its parts. The 
structure of the stratified drift (Fig. 511) shows that it was deposited 





Fig. 508. Stones of the drift, striated and beveled by glacial wear. (U. S. 
Geol. Surv.) 


by water, which doubtless sprang, in large part, from the melting of 
the ice. Either of the two great types of drift, the stratified and the 
unstratified, may overlie the other, or the two may be interbedded. 
The association of the two is such as to demonstrate their essential 
contemporaneity of origin. No agents but glacial ice and glacio- 
fluvial waters could have brought about such relations between the 
stratified and unstratified drift over such extensive areas. 

4. Distribution. The distribution of the drift is essentially the 
same as that of the ice-sheets and glacial waters; but apart from 
this general fact, several special features may be noted. (a) Within 
the area of its occurrence, the drift is measurably independent of 
topography. ‘That is, its vertical range is as great as the relief of 
the surface itself. Within the state of New York, for example, it 


CRITERIA OF GLACIATION 619 


ranges from sea-level to the tops of the Adirondacks, nearly 5,000 
feet above. It is found on hills and in valleys, and on plains, 
plateaus, and mountains, indiscriminately, though not usually in 
equal amounts. (b) Locally the drift is so disposed as to make the 
surface rougher than it would be otherwise, and in other places so 
as to give it less relief (Figs. 512 and 513). (c) In constitution it is 
measurably independent of present drainage basins. Thus, mate- 





Fig. 509. A section of unstratified drift, till or bowlder clay, on bed-rock. 
Newark, N. J. (N. J. Geol. Surv.) 


rials from one drainage basin are found in the drift of other drainage 
basins so commonly as to make it clear that present divides did not 
constitute divides to the ice. (d) Various sorts of material in the 
drift at certain points are so related to their sources as to make it 
clear that they were carried upwards, in some cases hundreds of 
feet, above their original sites. (e) A considerable area in south- 
western Wisconsin, and the adjacent parts of Illinois, Iowa, and 
Minnesota, is without drift. This driftless area is neither notably 
higher nor lower than its surroundings, and glacial ice seems to be 
the only agent which could have spared it, while covering its sur- 


620 THE PLEISTOCENE PERIOD 


toundings. (f) Stratified drift extends beyond the unstratified in 
the direction in which the ice was moving, especially i in valleys and 
on low land. This is the work of running water. 

5. Topography. Among the characteristic features of. ae 
topography of the drift are: (a) Depressions without outlets, and 
(b) associated knobs, hills, and ridges, similar in size to the depres- 
sions (Figs. 168 and 514). Many of the depressions contain ponds 





Fig. 510. Foliated till. (Photo. by Jefferson.) 


or lakes. The surface of some parts of the drift, on the other hand, 
is nearly plane. 

6. Thickness. The drift ranges from zero to more than 500 
feet in thickness, and the variations may be great within short 
distances. ‘The drift may be thick on hills and thin in valleys, or, 
more commonly, the reverse. No agent besides glaciers habitually 
leaves its deposits so unequally distributed, and in such disregard of 
pre-existing topography. 

7. Contact with underlying rock. The plane of contact be- 
tween the drift and the rock beneath is generally, though not always, 


CRITERIA OF GLACIATION 621 





Fig. 511. A section of stratified drift. 





Fig. 512. Diagram to show how drift may be so disposed as to increase the re- 
lief of the surface. This should be compared with the following figure. 





Fig. 513. Diagram to illustrate how drift may decrease ‘elief. 


622 THE PLEISTOCENE PERIOD 


sharply defined, and the surface of the rock is likely to be fresh and 
firm (Fig. 145). This relation is in contrast with that between 
mantle rock and the underlying formations where there is no drift 
(Fig. 152). 

8. Striation and planation.! The rock surface beneath the 
drift, and especially beneath the unstratified drift, is in many places 
polished, planed, striated (Fig. 145), and grooved. ‘These features 
are widespread throughout the drift-covered area, and they appear 





Fig. 514. Terminal moraine topography near Oconomowoc, Wis. 


at all elevations where there is drift. The strie on the bed rock 
beneath the drift are generally parallel in any given locality, and 
tolerably constant in direction over considerable areas; but when 
large areas are considered, the striz are in some places far from par- 
allel. Their direction corresponds with the direction in which the 
drift was transported. 

9. Shapes of rock hills. Many rock knolls which were left 
bare when the ice retreated show peculiarities of form and surface 
which are distinctive. ‘They were worn more on the side from which 
the ice approached (the stoss side) than on the other (Fig. 153). 
Bosses of rock which do not show notably unequal wear show dis- 
tinct smoothing. Projecting glaciated knolls of rock which show the 
characters seen in Fig. 167, p. 162, are known as roches moutonnées. 


1 Seventh Ann. Rept., U. S. Geol. Surv., has a full discussion of this topic. 


CRITERIA OF GLACIATION 





Fig. 515. The radiation of striz in the area of the Green Bay glacial lobe and 
in the west part of the Lake Michigan lobe, during the last glacial epoch. 


The true theory of the drift must explain all the foregoing facts 
and relations. Any hypothesis which fails to explain them all must 
be incomplete, and any hypothesis with which these facts and rela- 
tions are inconsistent must be false. Geologists are now agreed 
that glacier ice, supplemented by the agencies which it calls into 
being, is the only agent which could have produced the drift. This 
does not preclude the belief that at various times and places in the 
course of the ice period, icebergs were formed, or that locally and 


624 THE PLEISTOCENE PERIOD 





Fig. 516. Small protuberances of rock showing the effect of ice wear. Glacial 
knobs and trails. Movement of ice from left to right. The projections consist of 
chert in limestone. Near Darlington, Ind. (U.S. Geol. Surv.) 


temporarily they played an important réle. It does not preclude 
the idea that, contemporaneously with the production of the great 
body of the drift by glacier ice, the sea may have been working on 
some parts of the present land area, modifying the deposits made by 
ice and ice drainage. The glacial theory does not deny that rivers 
produced by the melting of the ice were an important factor in trans- 
porting and depositing drift, both within and without the ice- 
covered territory. It does not deny that lakes, formed in one way 
and another through the influence of ice, were locally important in 
determining the character and disposition of the drift. Not only 
does the glacier theory deny none of these things, but it distinctly 
affirms that rivers, lakes, the sea, and icebergs must have co-operat- 
ed with glacier ice in the production of the drift, each in its appro- 


> 


poo eo 


Fig. 517. Diagram to show the effect of ice wear on slight depressions in the 
surface of rock. 


priate way and measure, and that after the disappearance of the 
ice and the ice-water, the wind had some effect on the drift before 
it was clothed with vegetation. 


Development and Thickness of the Ice-sheets 


The development of glaciers from snow-fields has been dis- 
cussed (pp. 124-7). If the expansion of the ice-sheets was due prin- 


THICKNESS OF ICE-SHEETS 625 


cipally to movement from a center or centers, the ice at these centers 
must have been prodigiously thick, for in the course of its progress 
it encountered and passed over hills, and even mountains, of con- 
siderable height.. In the vicinity of elevations which it covered, its 
thickness must have been at least as great as the height of these 
elevations above their bases. 

If the centers of the North American ice-sheets remained the 
centers of movement throughout the glacial period, and if the degree 
of surface slope necessary for movement were known, the maximum 
thickness of the ice could be calculated. But it is probable that 
the centers of the ice-sheet did not remain the effective centers of 
movement. If the fall of snow toward the margin of the ice-sheet 
greatly exceeded that at its center, as it probably did, a belt near the 





Fig. 518. Diagram to illustrate the surface configuration of a great ice-sheet, 
according to the conception here presented. The central part is relatively flat, and 
the margins have steep slopes. 


margin, rather than the geographic center of the field, may have 
controlled the marginal movement of the ice. With excess of ac- 
cumulation near the border, the slope of the surface near the edge 
might be relatively great, while it was slight in the center of the field, 
as shown by Fig. 518. Under these conditions, the maximum 
thickness of the ice-sheets might be notably less than if the geo- 
graphic center remained the effective dynamic center. 

No sufficient data are at hand for determining with accuracy 
the average slope of such an ice-sheet as that which covered our con- 
tinent, but something is known of its slope at certain points. Near 
Baraboo, Wisconsin,! the edge of the ice at the time of its maximum 
extension in that region lay along the side of a bold ridge, the axis 
of which was nearly parallel to the direction of ice movement. The 
position of the upper edge of the ice against the slope of the ridge 
is sharply defined. For the last 134 miles, its average slope was 
about 320 feet per mile. This was at the extreme edge of the ice, 
where the slope was greatest. In Montana, the slope of the upper 
surface of the ice for the 25 miles back from its edge has been 
estimated at 50 feet per mile.’ 


1 Jour. Geol., Vol. III, p. 655. 
2 Calhoun, Jour. Geol., Vol. IX, p. 718. 


6206 THE PLEISTOCENE PERIOD 


The southern limit of drift in Illinois is not less than 1,500 or 
1,600 miles from the center of movement. An average slope of 
even 25 feet per mile for 1,600 miles would give the ice a thickness 
of 40,000 feet at the center, the slope of the surface on which the ice 
rested being disregarded. ‘This thickness seems incredible. Even 
an average slope of 10 feet per mile would give a thickness of about 
three miles at the center. If by reason of relatively great precipita- 
tion near its margins, the only part of the ice-cap which had con- 
siderable slope was its outer border (Fig. 518), a lesser maximum 
thickness would suffice. 

Stages in the history of an ice-sheet. The history of an ice- 
sheet which no longer exists involves at least two distinct stages. 
These are (1) the period of growth, and (2) the period of decadence. 
If the latter did not begin as soon as the former was completed, an 
intervening stage, representing the period of maximum ice ex- 
tension, is to be recognized. In the ice-sheets of the glacial period, 
each of these stages was probably more or less complex. The 
general period of growth was doubtless interrupted by short inter- 
vals of decadence, and the general period of decadence by brief 
intervals of growth. In the study of the work accomplished by an 
ice-sheet, it is of importance to distinguish between these main 
stages. 

Work of Ice-sheets 

Erosion and deposition were the two great phases of ice work 
(p. 147 et seq.). The surface over which the ice-sheets moved prob- 
ably had an erosion topography, and was covered by a layer of man- 
tle rock. ‘The ice removed the mantle of decayed material, and cut 
deeply into the undecayed rock beneath. By its erosion, the ice 
modified the topography to some extent, for weaker formations 
were eroded more than resistant ones, and topography favored more 
forcible abrasion at some points than at others. On the whole, the 
topographic effect of glacial erosion was probably to soften the sur- 
face contours, without diminishing the relief. 

The second great result of the ice-sheets was the deposition of 
the drift. Some of it was deposited while the ice-sheets were grow- 
ing, some of it after they had attained their growth, and some of 
it while they were declining. Some of it was deposited beneath 
the body of the ice, and some at its edge. Where it was thick, the 
drift altered the topography notably, especially where the relief 
of the underlying rock was slight. 


THE WORK OF ICE-SHEETS 627 


Formations made by ice-sheets.1 The drift formations fall 
chiefly into three categories, (1) those made directly by the ice (un- 
stratified), (2) those made by ice and water conjointly (stratified, 
but stratification more or less disturbed), and (3) those made by 
water emanating from the ice (stratified; cross-bedding common). 

Ground moraine (p. 159) is nearly co-extensive with the ice-sheets 
themselves, though it failed of deposition in some places, and has 


le iF "Ne he 


= 
;——4 
—* 
= 
oe 
= 
ra 
su 
au 
a 


Ie 





1 1/2 0 1 MILE 





Fig. 519. One phase of ground moraine topography; elongated hills of drift of 
the type shown, are called drumlins; southeastern Wisconsin. (U. S. Geol. Surv.) 


been removed in others. The ground moraine (till) of the North 
American ice-sheets is thickest in a broad belt a little within the 
margin of the drift (Fig. 504), extending from central New York 
through Ohio, Indiana, Illinois, Iowa, Minnesota, and the Dakotas, 
and thence northwestward. The topography of the ground moraine 
varies within wide limits. It is commonly undulatory, involving 
gentle swells and sags. In some places the swells take on rather defi- 
nite elongate shapes, with their longer axes in the direction of ice 
movement. They are then called drumlins (Fig. 519). Drumlins 
have pronounced development in eastern Wisconsin, where they 
are numbered by the thousand, in central and western New York, 
1 Jour. Geol., Vol. II, pp. 517-538, and Internat. Geol. Congr., 1893. 


628 THE PLEISTOCENE PERIOD 


in some parts of New England, and in some other places. The 
drumlins of New York are, in general, longer and narrower than 
those of Wisconsin. | 

The origin of drumlins has been much much discussed. Opinion 
is divided chiefly between the views (1) that they were accumulated 
beneath the ice under special conditions, and (2) that they were — 
developed by the erosion (by the ice) of earlier aggregations of drift.! 

A terminal moraine (p. 149) may be very like the adjacent ground 
moraine in constitution, though in many places there is more strati- _ 





Fig. 520. A Wisconsin drumlin seen from the side; two miles north of Sullivan. 
(Alden, U. S. Geol. Surv.) 


fied drift associated with it. It commonly constitutes something of 
a ridge, but it is more accurately characterized as a belt of thick 
drift. Its most distinctive feature does not le in its importance 
as a topographic feature, but in the details of its own topography. 
Its surface is, as a rule, characterized by hillocks and hollows, or 
by interrupted ridges and troughs (Figs. 168 and 514). Many 
of the hollows and troughs contain marshes, ponds, and lakes. The 
shape and abundance of round and roundish hills, and of short 
and more or less serpentine ridges closely huddled together, have 
given rise locally to such descriptive names as ‘‘knobs,” “‘short 
hills,” etc.; but it is the association of ‘‘knobs” or ‘‘short hills” 

1Some of the more important papers on drumlins are: Upham, Proc. Bos. 
Soc. Nat. Hist., 1879, pp. 220-234, ibid., Vol. XXIV (1889), pp. 228-242; Chamber- 
lin, Third Ann. Rept., U. S. Geol. Surv., 1883, p. 306, and Jour. Geol., Vol. I, 
pp. 255-267; Davis, Am. Jour. Sci., Vol. XXVIII (1884), pp. 407-416; Salisbury, 
Glacial Geology of New Jersey, 1902; Lincoln, Am. Jour. Sci., Vol. XLIV (1892), 


pp. 293-296; Tyrrell, Bull. Geol. Soc. Am., Vol. I (1890), p. 402; Leverett, Monogrs. 
XXXVIII and XLI, U. S. Geol. Surv., and Russell, Amer. Geol., Vol. XXXV_ 


(1905), Pp. 177. 


, TYPES OF DRIFT 629 
with ‘‘kettles,” and not either feature alone, which is characteristic 
of 


terminal moraine topography. 
The ‘‘knobs” vary in size, from low mounds but a few feet 








across, to hills half a mile or more in diameter, and a hundred feet 
or more in height. Not rarely they are about as steep as the mate- 
rial of which they are composed will lie. The ‘‘kettles” are the 
counterparts of the elevations. They may be a few feet, or many 


630 THE PLEISTOCENE PERIOD 


rods, or even furlongs in diameter. They may be so shallow that 
the sagging at the center is hardly seen, or they may be scores of 
feet in depth. Where steep-sided depressions are closely associated 
with abrupt hillocks, the topography is notably rough. The topog- 
raphy of the terminal moraine may be well developed, even where 
the moraine as a whole does not constitute much of a ridge. 

The surface of the terminal moraine, where well developed, is 
generally rougher than that of the ground moraine, but because the 


g 
8 
- 


vi 


S 
way 
(Oy 
in w 
‘ANS 
\' } 2 
kA \ 
. > ’ 
S \\ 
SS} 43 
RS YW 
J SAL <5) 
ah. | 
& gear 


~\ Wh 


Ne 





Fig. 522. ‘Topography of drift shown in contours; an area near Minneapolis, 
Minn. Scale about one inch to the mile. (U.S. Geol. Surv.) 


sags and swells are of smaller area and steeper slopes, rather than 
because the relief is notably more. It is not to bé understood, 
however, that the topography described affects all terminal mo- 
raines, or that it is confined strictly to them. The elevations and 
depressions of terminal moraines grade from strength to weakness, 
and locally even disappear, while the features characteristic of ter- 
minal moraines are found, now and then, in other parts of the drift. 

1 References to papers on terminal moraines: Chamberlin, Third Ann. Rept., 


U.S. Geol. Surv., 1881-2, pp. 291-402, and Amer. Jour. Sci., Vol. XXIV (1882), — 
pp. 93-97; Salisbury, Glacial Geology of New Jersey, pp. 92-100 and 231-260. 


TYPES OF DRIFT 631 


Where an_ice- 
sheet halted in 
its retreat, its edge 
remaining in a 
constant or nearly 
constant position 
for a sufficiently 
long period, a ter- 
minal moraine 
(called a recessional 
moraine) was de- 
veloped. The not 
uncommon impres- 
sion that a terminal 
moraine necessarily 
marks the terminus 
of the drift is erro- 
neous. The word 
terminal refers to 
the terminus of the 
ice at the time 
when it formed the 
moraine. 

Fluvio-glacial de- 
posits have been re- 
ferred to in earlier Fig. 523. A group of kames near Connecticut Farms, 
pages(p.164). They N.J. (N. J. Geol. Surv.) 
are made (1) at the 
edge of the ice (kames and various ill-defined accumulations of 
gravel and sand); (2) beyond the edge of the ice (valley trains, 








Ord 1, 00 
be Ts Nt ES es es 


Fig. 524. Diagram to illustrate kame terraces. ABC represents the stratified 
drift of the kame terraces which are underlain by ground moraine. Till also covers 
the valley bottom. 


outwash plains, deltas and various ill-defined bodies of stratified 
drift); and beneath the ice (eskers, etc.). 


632 THE PLEISTOCENE PERIOD 
Changes in Drainage Effected by Glaciation 


One result of the unequal erosion and unequal deposition by the 
ice-sheets was the derangement of drainage. This is seen in the 
thousands of lakes, ponds, and marshes which affect the surface of 
the drift. The basins of the lakes or ponds arose in various ways. 
There are (1) rock basins produced by glacial erosion; (2) basins 
due to the obstruction of river valleys by drift; (3) depressions in 
the surface of the drift itself; and (4) basins produced by a combi- 
nation of two or more of the foregoing. Besides the lakes and ponds 
now in existence, others have become extinct by the filling of their 
basins or by the lowering of their outlets. 

Glaciation also changed the courses of many streams. In many 
cases, pre-existing valleys were filled with drift in some places, so 
that when the ice melted, the drainage followed courses which were 
partly new. In other cases, the ice forced streams to flow around 
its edge, and some of the drainage channels thus established were 
held after the ice melted. There are few streams of great length in 
the area covered by the ice which were not turned from their old 
courses for greater or less distances by the ice or the drift which the 
ice left. The Mississippi, the Ohio, and the Missouri, the master 
streams of the United States within the glaciated area, and a host 
of their tributaries, suffered in this way.! 


Succession of Ice Invasions 


The glaciation of North America was accomplished by a se- 
ries of ice-sheets separated from one another by long intervals of 
time. Some of the interglacial intervals were much longer than the 
time since the last ice-sheet disappeared, and there is also good 
evidence that in some of them the climate was at least as mild as 
to-day. 

The proofs of the interglacial intervals and the evidences of 


1 For changes in the Mississippi and in the rivers of Illinois, see Leverett, 
Monogr. XXXIII, U. S. Geol. Surv., p. 120. For changes in the Upper Ohio, 
see Chamberlin and Leverett, Am. Jour. Sci., Vol. XLVII, 1894 (contains refer- 
ences to earlier work in this region). For changes in the Erie and Ohio Basin, 
see Leverett, Monogr. XLI, U. S. Geol. Surv., Chap. III, and Tight, Professional 
Paper No. 13, U. S. Geol. Surv. For changes in the course of the Upper Missouri 
and its tributaries, see Todd, Science, Vol. XIX, p. 148 (1892), Geol. of S. Dak., 
pp. 128 and 130 (1899), and Bull. 144, U. S. Geol. Surv. Changes in drainage 
in New York have been summarized by Tarr, Phys. Geog. of New York, 1902, 
with references to earlier literature. 


SUCCESSION OF ICE-SHEETS 633 


their duration are found (1) in the erosion effected by streams after 
the deposition of one sheet of drift and before the deposition of 
the next, (2) in the depths to which earlier sheets of drift were 
leached and oxidized by weathering before the deposition of later 
ones upon them, (3) in the accumulations of peat, soil, etc., now 
‘found between different sheets of drift, and (4) in the changes of 
topographic attitude which intervened between the deployment of 
successive ice-sheets.! 

The following are the stages of the glacial period recognized in 
North America numbered in the order of their age: 

VIII. The Glacio-lacustrine (including the Champlain). 

VII. The Wisconsin or Wisconian, the last important invasion. 
VI. The Sangamon-Peorian, or third interglacial interval. 

VB. The Iowan, the third invasion in the Keewatin field. 

(VA. The Illinoian, the third invasion from the Labradorean field. 
IV. The Yarmouth or Buchanan,’ the second interglacial interval. 
III. The Kansan, or second ice invasion. 

II. The Aftonian, or first interglacial interval. 

I. The Jerseyan or sub-Aftonian ice invasion, the earliest recog- 

Peenized. 

I. Jerseyan or Sub-Aftonian glacial stage. The oldest drift 
which appears in New Jersey is but the frayed edge of a once con- 
tinuous sheet, and is very old. On the Allegheny and upper Ohio 
rivers, the great age of the oldest drift is shown by the deep erosion 
of the valleys since the first ice invasion turned the streams into new 
channels. Farther westward the corresponding old drift is covered 
by later drift. In the Keewatin area in Iowa, a very old drift (sub- 
Aftonian), probably the equivalent of the Jerseyan, lies below the 
Aftonian and Kansan. Very old mountain drift has recently been 
found (Atwood) high on the mesas near the San Juan Mountains in 
Colorado and also on the high mesas in front of the Rocky Moun- 
tains in Montana (Alden). The evidences of age of all these seem 
to be of the same order, and they are thought to represent the 
earliest ice invasions in the Labradorean, Keewatin, and Cordil- 
leran fields. 

II. Aftonian interglacial interval. Overlying the oldest till in 


1Distinct glacial epochs and criteria for their recognition, Jour. Geol., Vol. I, 
pp. 61-84. 

2 The Buchanan gravels lie between the Kansan and Iowan drift-sheets where 
the Illinoian is not present, and hence their age is not quite certain. 


634 THE PLEISTOCENE PERIOD 


Iowa is an irregular sheet of sand and gravel with remnants of old 
soil, muck, and peat, with stumps and branches of trees. The sur- 
face of the drift beneath shows much weathering and erosion. The 
fossils in these interglacial beds imply a cool-temperate climate; 
but as a cool-temperate stage must be passed through twice between 
successive glacial epochs, once as the ice retreats, and a second 
time as it advances again, fossils indicating a cool climate do not 
necessarily show how warm the interglacial epoch may have become. ~ 

III. Kansan glacial stage. The Kansan stage is represented by 
a sheet of till occupying a large surface area in Kansas, Missouri, 
Iowa, and Nebraska. ‘Theoretically it extends under the later gla- 
cial formations to the northward, as far back as the Keewatin 
center of radiation. Much of this sheet of drift, as originally devel- 
oped, probably was rubbed away by later glaciations. Presumably 
a similar sheet was formed by a contemporaneous ice-sheet spread- 
ing from the Labrador center, but it has not been certainly identi- 
fied. The Kansan till is clayey and there is little stratified drift 
associated with it. 

IV. The Yarmouth interglacial stage.! Where the Illinois till 
overlaps the Kansan (eastern Iowa), an old soil, with deep subsoil 
weathering, lies on the surface of the latter. 

V. Illinoian and Iowan glacial stages or Iowa-Illinoian stage. 
On the borders of the Labradorean field near the Mississippi River, 
the Illinoian drift sheet overlies the Kansan sheet with the Yarmouth 
beds between. In the Keewatin field in eastern Iowa, the Iowan 
drift lies over the Kansan, with the Buchanan beds between. 
Some geologists now think that the Iowan represents the same stage 
in the Keewatin field that the Illinoian does in the Labradorean 
field; i. e., the third ice invasion. The earlier view was. that the 
Illinoian drift was the older. 

V. A. Illinoian drift sheet (Labradorean field). The exposed por- 
tion of this drift occupies the surface in the southern and western 
parts of Illinois. It runs back under the later drift to the north- 
east toward the Labradorean center. ‘To the eastward, it is traced 
as far as Ohio, where it is covered by later drift. To the northward 
its margin is covered in southern Wisconsin, but in central Wis- 
consin it seems to re-appear and is traced westward on the north 
side of the driftless area, beyond which, in Minnesota, it seems to 
connect with the Iowan drift. The Illinoian till is clayey, with little 

1 Leverett, Mono. XXXVIII, U. S. Geol. Surv. 


SUCCESSION OF ICE-SHEETS 635 


assorted drift associated. The west edge of the Illinoian ice-lobe 
pushed out into Iowa a score of miles, forcing the Mississippi in 
front of it. Ice of the Kansan epoch had earlier invaded Illinois 
from the west, and probably forced the Mississippi east of its 
present course, if such an easterly course had not been taken be- 
fore the Kansan epoch. 

V. B. The Iowan drift (Keewatin field). In northeastern Iowa 
the ice of this stage left a thin sheet of till marked by a profusion of 
large granitoid bowlders most of which lie on the surface. To the 
northward in Minnesota these bowlders are less abundant, and the 
formation passes beneath later drift. To the northeast it appears 
to be connected with the third drift of Wisconsin. 

VI. Sangamon interglacial stage. In central Illinois a sheet 
of sandy material marked by remnants of old soil, muck, peat, 
weathering and erosion overlies the Illinoian glacial drift. Above 
this lies a mantle of loess and the Peorian peaty beds. According 
to the older view, the Iowan was placed between the Sangamon 
and the Peorian, now regarded, tentatively, as equivalents. 

VII. Wisconsin or Wisconian stage. The ice radiated from the 
Labradorean, Keewatin, Cordilleran, and from many mountain 
centers. It had probably done this at each of the preceding glacial 
stages, but the record is much obscured by erosion and concealment. 
The margin of the Wisconian ice was pronouncedly lobate, and the 
drift which it left is characterized by stout terminal moraines, 
numerous kames, eskers, drumlins, outwash aprons, valley trains, 
and other features distinctive of glacial action and glacio-fluvial 
coéperation. This drift-sheet, more than any of the others, bears 
the stamp of the great agency of the period. The distinctive topog- 
raphy of the various phases of this formation is in contrast with the 
relatively expressionless surfaces of the older sheets of drift. Part 
of this difference is due to the fact that the Wisconsin formation has 
been eroded less than the older drifts; but the larger part, apparently, 
is assignable to a stronger original expression. 

Unlike the earlier sheets of drift, the Later Wisconsin drift was 
not overriden by later sheets of ice, and its original development 
is therefore better shown at the surface. It bas nearly a score of 
concentric terminal moraines in some places. Some of them repre- 
sent re-advances of the ice in the course of its general retreat, while 
others mark halts in the retreat sufficient to permit an exceptional 
accumulation of drift at the border of the ice, 


636 THE PLEISTOCENE PERIOD 


Not all of these several sheets of drift have been seen in super- 
position, and the history sketched above is based on the relations of 
the sheets of drift at different points.1 Theoretically, the several 
sheets of drift are imbricated as suggested by Fig. 525, but each 
sheet of drift is discontinuous beneath the overlying one, and this 








—_ SS eee eee eT ll OTC OTC — comer que oe ow es ons Oe oe oS ee 


Fig. 525. Diagram illustrating the imbrication of the successive sheets of 
drift. The full lines represent the portion of the drift-sheets not overspread 
by later ice-sheets; the broken lines represent the portions of the successive drift- 
sheets which were covered by ice at a later time. 1 corresponds to Jerseyan 
or sub-Aftonian, which in general is less extensive than the Kansan, though 
locally, as in New Jersey, it extended farther south than any other; 2 repre- 
sents the Kansan drift, the southern margin of which is not covered by younger 
drift; 3 and 4, respectively, represent the Illinois-Iowan, and Wisconsin 
sheets of drift. 


discontinuity goes so far that beneath the Wisconsin drift, for ex- 
ample, the several sheets are more commonly wanting than present. 

VIII. Glacio-lacustrine stage. In the course of the retreat of 
the ice of the Wisconsin epoch, a complex series of lakes arose be- 
tween the ice border on the one hand, and the higher land fronting 
it on the other. Many of these lakes were temporary and shifting, 
and had shifting outlets. Their history cannot be given here; but 
a brief sketch of the history of the Great Lakes will indicate the na- 
ture of the changes which took place. 

When the end of the Lake Michigan ice-lobe (Fig. 526) with- 
drew a little from the southern end of the Lake Michigan basin, a 
lake formed there, and discharged its waters into the Illinois valley — 
southwest of Chicago. The channel followed by the outflowing 
waters has since become the site of the Chicago drainage canal. 
The glacial lake (Lake Chicago) thus initiated was gradually extend- 
ed northward (Fig..527) as the ice-lobe was melted. 

A similar lake was formed about the head of the Lake Superior 
ice-lobé. Lake Maumee developed about the end of the Erie ice- 
lobe, and its waters flowed to the Wabash. A later stage of Lake 
Chicago and Lake Maumee is shown in Fig. 527, when, finding a 
lower outlet as the ice melted back, Lake Maumee sent its outflow 
across southern Michigan to Lake Chicago. 

Later, the whole Erie basin, and a portion of that of Ontario, 

1 Jour. Geol., Vol. I, pp. 61-84. 


GLACIO-LACUSTRINE STAGE 63% 


~Nyhaee? 


ate 

aes 
-e., 
‘a, 


x 
Ps ras 

- eat 
ate 

« 

‘ 


SNS — — 





Fig. 526. The beginning of the Great Lakes. The ice still occupied the 
larger parts of the present lake basins. (U.S. Geol. Surv.) 





ris aaa 
“<ghe MCLE 
, abe \\ 
fag ey Tae 
ie¥ 


en) 

<ose 
y om 
——— KOOPi lay 


\\ 


Fig. 527. A later stage in the development of Lakes Chicago and Maumee. 
The ice has retreated, and the outlet of Lake Maumee has been shifted. (U.S. 
Geol, Surv.) 


638 THE PLEISTOCENE PERIOD 


was freed of ice, and a lake (Lake Arkona) twice as large as Lake 
Erie developed. An advance of the ice changed the lake and with 
its changed outline it is known as Lake Whittlesey (Fig. 528.) 





Fig. 528. A later stage in the development of Lakes Chicago, Maumee, and 
Saginaw. (Leverett and Taylor, U. S. Geol. Surv.) 


6 Sf Foes ! Vi 
rio“ Lobe tA 
eofeo- 4, / 1 wy ON 





Fig. 529. Illustrating the relations of standing water to the ice in the Erie and 
Ontario regions after the ice had retreated farther than represented in Fig. 528, 
The numerous lobate arms of lakes south of the Ontario lobe of ice will be noted, 
and also the fact that the discharge of Lake Warren was still to Lake Chicago. 
(Taylor and Leverett, U. S. Geol. Surv.) 


With further retreat of the ice, the ponded waters of the region 
assumed the form shown in Fig. 529. At first, this lake discharged 
across Michigan into Lake Chicago, but later, when the Mohawk 


GLACIO-LACUSTRINE STAGE 639 


valley was freed from ice, it offered the lower outlet, and the level of 
Lake Warren was drawn down, and it was divided into two lakes, 
Erie and Iroquois (Fig. 530). 

Meantime, the glacial lakes in the basins of Lakes Michigan and 
Superior experienced analogous shiftings of areas and of outlets. 
While Lake Iroquois was discharging through the Mohawk valley, 
Lake Algonquin (Fig. 530), was discharging its waters eastward. 
At first the outlet was probably by the St. Clair-Erie route, through 





Pigwess..) Lhe Great Lakes at the Algonquin-Iroquois stage. The outlet to 
the sea is by way of the Mohawk Valley. (Taylor.) 


Lake Iroquois, to the Mohawk; but later, when the ice had retired 
farther north, an outlet appears to have been opened from Georgian 
Bay to Lake Iroquois, by way of the Trent River. 

When at length the ice withdrew from the Adirondacks so far as 
to permit the waters of Lake Iroquois to find an outlet lower than 
that by way of the Mohawk, a new series of lowerings of the lakes 
followed. At first the outlet seems to have skirted the Adirondacks 
and emptied into a glacially-ponded water-body (glacial Lake 
Champlain) which occupied the Champlain basin, and discharged 
southward through the Hudson. Later Lake Algonquin gave place 
to the great Nipissing Lakes (Fig. 531), which had their outlet via 
Lake Nipissing to the Ottawa, and thence to the Champlain arm of 


640 THE PLEISTOCENE PERIOD 


the sea. Subsequently the outlet was shifted to its present position, 
probably by a gentle upwarping of the surface at the north. 

Similar complicated histories doubtless attended the retreat 
of the ice in the Mackenzie and Hudson Bay basins, but little is yet 
known regarding them. A very important lake was formed in the 
Red River valley of the North (Lake Agassiz, Fig. 532), discharging, 


SI. PG 





S 
S 


Mi pb 


z 
3a 
My 
Y; 


Le 


Fig. 531. Acstill later stage of the Great Lakes. The sea is thought to have 
covered the area shaded by lines at the east. (Taylor.) 


in its earlier history, into the Minnesota River at Lake Traverse. 
Lake Agassiz had a comparatively simple history. It grew to the 
northward with the retreat of the ice which held it in at that end, 
and continued to discharge into the Minnesota River until the 
retreat of the ice gave it a northerly outlet. It developed beaches 
while it discharged to the southward, and another set after the out- 
let was northward. On the final withdrawal of the ice, the lake was 
drained. 

The evidence which demonstrates the existence of these ex- 
panded lakes is found chiefly in the deposits which they made, and 
in the topographic features which they developed about their shores: 
Many of the former shore-lines have been traced in detail, and most 
of them depart notably from horizontality. In general, they rise 


GLACIO-LACUSTRINE STAGE 641 


to the north and northeast. It is probable that there were corre- 
sponding lacustrine substages at the close of each of the several 
giacial epochs, but their history is not known. 

In the later part of this substage, an arm of the sea extended up 
the St. Lawrence to Lake Ontario, filling the basin of Lake Cham- 
plain (Fig. 531). It probably connected southward by a narrow 





Fig. 532. Map of the extinct Lake Agassiz, and other glacial lakes. Lake 
Winnipeg occupies a part of the basin of Lake Agassiz. (U.S. Geol. Surv.) 


strait along the site of the Hudson valley with the ocean. The 
sediments deposited in this arm of the sea contain shells and bones 
of marine animals. ‘The marine fossils are found at various places 
about Lake Champlain at altitudes varying from 400 feet or less 
about the south end of the lake, to 500 feet at the north end, and 
about 600 feet near the east end of Lake Ontario.! At about the 
same time the sea stood higher than now relative to the land on the 


4 Dawson, G. M., Am. Jour. Sci., 3d ser., Vol. VIII (1874), p. 143; Dawson, 
J. W., The Canadian Ice Age, p. 201, and Am. Jour. Sci., Vol. CXXV, 1883. 


642 THE PLEISTOCENE PERIOD 


coast of Maine, where marine shells occur up to elevations of 200 
feet or more,! and to still greater heights farther north. 


Loess 


The term loess is used both as a textural and a formational name. 
Lithologically, it is a silt intermediate between sand and clay. It 
is generally free from stones of all sorts except concretions developed 
in it since its deposition. In the exceptional cases where stones 
occur in it, they are confined in most cases to its very bottom, or to 
loess which has slumped or been washed down from its original posi- 
tion. It is interstratified with sand in some places. 

Composition. The loess contains many angular, undecomposed 
particles of the commoner carbonates (calcite and dolomite) and 
. silicates (feldspars, amphiboles, pyroxenes, micas, etc.), and a few 
of the rarer silicates. Magnetite also is a common, though never an 
abundant, constituent. All these are subordinate to quartz. These 
constituents strongly suggest that the material of the loess was 
derived from the rock-flour of the drift. In color it is generally 
buffish, but in not a few places it has a grayish (bluish) cast a few 
feet below the surface. 

Loess stands readily with vertical faces (Fig. 533) for long 
periods, where sand or clay would be degraded into slopes. Roads 
on the loess tend to assume the form of little canyons, because the 
silt of the road-bed is washed or blown away, while that on either 
side stands up with steep or even vertical slopes. Many weathered 
faces of the loess show a rude columnar structure (Fig. 533), the 
columns being one to several feet in diameter. The loess, as a rule, 
shows no stratification, but in its coarser phases there is some sug- 
gestion of such structure, and where interbedded with sand, strati- 
fication may be distinct. 

Distribution. The best known loess in America and Europe is 
associated with glacial drift, though loess extends far beyond the 
borders of the drift in some directions, in both continents. In 
China and other lands of Asia, where loess has great development, it 
is not generally associated with glacial formations. 

In North America the loess does not occur east of the Mississippi 
basin, and has little development east of the Wabash River. It 
is widespread in Illinois and the states along the Missouri, and in 


1 Stone, Jour. Geol., Vol. I, pp. 246-254, and Bastin, Rockland, Me., folio, 
U. S. Geol. Surv. 


LOESS 643 


the states along the Mississippi farther south. Within this area, 
its distribution is peculiar in that it follows the main streams, and 
is found especially on the bluffs overlooking the valleys. On this 
account it was formerly known as the Bluff formation. In this 
bluff-position, it has more than its average thickness and coarseness 





Fig. 533. A section of loess in Iowa, showing its ability to stand with vertical 
or even overhanging faces. (Calvin.) 


of grain. It grows thinner and finer in grain back from the river 
bluffs, until it is lost in a vanishing edge. As it thins, its material 
loses its distinctive characteristics. 

South of the borders of the Illinois-Iowan and Wisconsin drift- 
sheets, it mantles many of the divides between the main streams; but 
farther south it is confined more to the valley borders. Within the 
drift-covered part of the Mississippi basin, it occurs (1) as a surface 
mantle overlying drift, and (2) between sheets of drift. South of 
the drift there are in places (e.g. southern Illinois and northeastern 
Arkansas) two distinct sheets of loess, separated by a well developed 
soil zone. ‘The surface of the lower sheet shows the effects of pro- 


644 THE PLEISTOCENE PERIOD 


longed weathering and oxidation, in some places. Loess occuts in 
isolated spots even as far west as Washington and Oregon. 

Age. The relations of the loess to the several drift-sheets make 
it clear that it was accumulated at different stages of the glacial 





n 0 P q 
Fig. 534. Loess Shelis. a-b, Zonitoides minusculus (Binney); c-d, Euconulus 
fulvus (Drap.); e-f, Strobilops labyrinthica (Say); g, Polygyra clausa (Say); h, P. 
multilineata (Say); i-7, Succinea obliqua Say; k, S. avara Say; l-m, Polygyra monodon 
(Rack); , Bifidaria pentodon (Say); 0, B. corticaria (Say); p, B. muscorum (Linn.); 
q, B. armifera (Say). The small figures adjacent to some of the large ones show the 
natural size of the shells. 


period, but within the glaciated area most of it is younger than 
the Illinoian sheet of drift which it mantles, and older than the 
Wisconian drift which overlies it. Locally, loess covers Wisconsin 
drift in a few places. No considerable body of loess older than 
the Illinois drift has been identified with certainty. | 


LOESS 645 


Thickness. The loess of the Mississippi basin rarely is more 
than a score or two feet thick, and this only along the main valleys; 
but exceptionally its thickness approaches 100 feet. Thicknesses of 
10 feet are much more common than greater ones. 

Accessories. The loess contains characteristic accessories of 
two kinds, concretions and fossils. The concretions are of lime 
carbonate and iron oxide. Many of the former are irregular, 
and of such shapes as to have been called ‘‘petrified potatoes”’; 
but many of them have other shapes. The ferruginous con- 
cretions take various forms, one of which is the ‘‘pipe stem,” 
perhaps formed about rootlets. The fossils are chiefly gastropods 
(Fig. 534), almost wholly of land species, or of such as frequent 
isolated ponds. The other fossils are bones and teeth of land 
mammals. 

Origin. There has been much diversity of opinion as to the 
origin of loess, the fundamental question being whether it is aqueous 
or eolian. There is little doubt that the loess-like silts which occur 
in the terraces of rivers are of fluvial origin; but some would not 
regard them as loess. Some, indeed, would restrict the term to an 
eolian product. 

There is a growing conviction that most of the loess on the 
uplands, in the United States at least, is eolian. The river flats 
are supposed to have supplied much of the material of the loess, the 
alluvial silt being whipped up by the winds and re-deposited on the 
adjacent uplands. The rivers are thus made essential factors in its 
distribution, though not the direct agents of deposition. This hy- 
pothesis seems on the whole best to fit the phenomena, of the larger 
part of the upland loess of the Mississippi basin. The constituents 
of the loess, which appear to have come from the glacial drift, were 
derived largely from the deposits made by glacial waters, or from 
later flood plain silts derived from the glacial formations; but it is 
probable that some of the loess was derived from glacial drift direct- 
ly, before it became clothed with vegetation.! 

1 References. Loess is described in the geological reports of many of the 
states of the central Mississippi basin. Other references are McGee, Eleventh 
Ann. Rept., U. S. Geol. Surv.; Chamberlin and Salisbury, Sixth Ann. Rept., U. S. 
Geol. Surv.; Shimek, Am. Geol., Vols. XXVIII and XXX, Bull. Ia. Lab. Nat. 
Hist., Vols. I, II, and V, Proc. Ia. Acad. Sci., Vols. III, V, VI, and VII; Leverett, 
Am. Geol., Vol. XX XIII, and Monog. XXXVIII; Calvin, Bull. Geol. Soc. Am., 


Vol. X, p. 119; Chamberlin, Jour. Geol., Vol. V, 1897; Davis, Explorations in 
Turkestan, 1905; and Willis, Researches in China, Vol. I, Carnegie Institution. 


646 THE PLEISTOCENE PERIOD 


Duration of the Glacial Pertod 


The desire to measure the great events of geological history in 
terms of years increases as our own time is approached. The un- 
certainties attending such measurements are, however, so great 
that the results have an uncertain value, and do little more than 
indicate the order of magnitude of the time involved. Attempts 
have been made (1) to estimate the relative duration of the several 
glacial and interglacial epochs, and (2) to estimate in years the time 
since the close of the glacial period. 

1. The best data for estimating the relative duration of the 
several glacial stages are found in the central basin of the Mississippi, 
for here only are all members of the drift series present. The 
criteria that have been used in estimating relative duration embrace 
(1) the amount of erosion of the several drift sheets, (2) the depth of 
leaching, weathering, and decomposition of its materials, (3) the 
amount of vegetable growth in interglacial intervals, (4) the climatic 
changes indicated by interglacial and glacial floras and faunas, (5) the 
time needful for the migration of faunas and floras, particularly 
certain plants whose means of migration are very limited, (6) the 
time required for advances and retreats of the ice,-and some others. 
A few of these, as the first, are subject to direct measurement; but 
most of them are matters of judgment. By the use of these data, 
it has been estimated that the time since the Kansan drift was 
deposited is some 15 to 20 times as long as the time since the last 
glacial epoch. 

2. Of the efforts that have been made to measure in years post- 
glacial time, those based on the recession of Niagara and St. Anthony 
Falls are the most significant.! In both these instances, the meas- 
urement attempted is the time occupied in the recession of the falls 
from their starting point to their present positions. 

If the length of the Niagara gorge be divided by the average an- 
nual retreat since the falls were first located by accurate surveys, 

' References on Niagara: Gilbert, Am. Jour. Sci., 3d ser., Vol. XXXII, 1886; 
Science, Vol. VIII, 1886; Chapter in Physiography of the United States, and Bull. 
U.S. Geol. Surv. Upham, Am. Jour. Sci., 3d ser., Vol. XLV; Jour. Geol., Vol. I. 
1893; Am. Geol., Vol. XI, 1893, and XVIII, 1896, and Pop. Sci. Mo., Vol. XLIX, 
1896. Spencer, Evolution of the Falls of Niagara; Taylor, Bull. Geol. Soc. Am., Vol. 
IX, p. 84, and Vol. XXIV. 

St. Anthony Falls: Winchell, N. H. Fifth Ann. Rept. Natl. Hist. and Geol. 


Surv. of Minn., 1876; Geol. of Minn., Vol. II, 1888, 23d Ann. Rept., 1894; Southall, 
The Epoch of the Mammoth, p. 373. 


DURATION OF GLACIAL PERIOD 647 


the quotient is about 7,000, but it is not safe to assume that this 
number of years is the time since the last glacial epoch. At the 
beginning of the cutting of the gorge, the waters of the upper lakes 
flowed by a more northerly route to the sea (Figs. 530 and 531), 
leaving only the waters of the Erie basin to pass over the falls. If 
the history is correctly read, it was at a comparatively late date that 
the waters of the Upper Great Lakes went out through the Niagara 
River. The early cutting was therefore much slower than the later. 
In view of these considerations, it is thought that 7,000 should be 
multiplied several times to give the true time-estimate. Spencer 
places the period at about 39,000 years, and Taylor at about 25,000 
years. 

It is to be noted that cutting of the Niagara Gorge could not have begun until 
the Mohawk outlet of the lakes (p. 639) was abandoned, and that the time measured 
by the Niagara cutting is only that which has elapsed since the ice melted back 
from the Adirondacks far enough to permit the waters of the ancestral Lake 
Ontario to find an outlet lower than the Niagara escarpment, and no very effective 
cutting could take place until the waters were withdrawn to something near their 
present level. 

If the border of the ice-sheet at this stage (Fig. 531) is compared with the 
border of the ice at the maximum Wisconsin stage, it will be seen that it had re- 
treated seme 600 miles. The rate of recession of the ice is unknown, but 200 feet 
per year is an improbably high rate; but at this rate, the ice must have been reced- 
ing some 15,000 years, before the falls came into existence. If this be added to the 
time occupied in the development of the gorge, say 25,000 to 40,000 years (esti- 
mated), the result is 40,000 to 55,000 years since the beginning of the retreat of the 
last great ice-sheet. 


From a comparison of the earlier and later surveys of St. Anthony 
Falls, the time of recession of the falls from the mouth of the gorge 
has been estimated at about 8,000 years. But considerations not 
taken into account in this estimate make it clear that this estimate 
should be increased to 12,000 or 15,000 at least. If to these figures 
20,000 years be added for the time of retreat (700 or 800 miles) before 
the falls began to develop, we have a total of more than 30,000 
years since the climax of glaciation in the late Wisconsin epoch. 

Little value is to be placed on estimates of this kind, except as 
means for developing a conception of the order of magnitude of the 
time involved. 


Foreign 


In Europe, the succession of ice epochs and formations is not less 
complex than in North America, though there is not complete agree- 


648 THE PLEISTOCENE PERIOD 


ment among geologists as to the number of glacial epochs.!_ In the 
Alps four are recognized.2, These are designated * Giinz (pre- 
Kansan?), Mindel (Kansan?), Riss (lowa-Illinoian), and Wiirm 
(Wisconsin?). The glacial formations of other continents have 
not been studied in detail in many places, but recent studies in 
Turkestan indicate that there were several glacial epochs in the 
Thian Shan Mountains.* 


CAUSE OF GLACIAL CLIMATE 


Many hypotheses of the cause of the glacial period have been 
offered, but none commands universal assent. Most of them appeal 
to a combination of agencies, but each centers on some one factor 
which gives character to the hypothesis. They fall mainly into 
three classes: (1) those based on elevation of the land, the hypso- 
metric hypotheses; (2) those based on phenomena and relations out- 
side the earth itself, the astronomic hypotheses, and (3) those based 
on changes in the constitution, movements, or cloud-content of the 
air, the atmospheric hypotheses. 

Hypothesis of elevation.® Since the best-known glaciers are 
in mountains, the suggestion was natural that elevation of the 
glaciated regions was the cause of the great ice-sheets. The chief 
evidence of the elevation postulated is the submerged valleys of the 
sea-coasts, especially those of the northern latitudes. It has been 
held by advocates of this hypothesis that 4,000 feet or more of eleva- 
cion is indicated by the northern fiords, and that this elevation, to- 
gether with accompanying geographic changes, was competent to 
produce the Pleistocene glaciation. Those who question this view 
doubt the fact of so great elevation, and doubt whether any eleva- | 
tion which there may have been was contemporaneous with the 
ice-sheets. Further, they offer evidence that the land was lower 
than now at certain important stages of the glacial period. The 
elevation hypothesis also encounters grave difficulty in explaining 
the repetition of glacial epochs and interglacial epochs, and in 
accounting for the mild climates of interglacial times. In its simple 

1 Geikie, Jour. Geol., Vol. III, pp. 241-269. Keilhack, ibid., vol. ITI, 
Pp. 113-125. 

2 Penck, Die Alpen im Eiszeitalter. 

3 Penck, Science, Vol. XXIX, p. 350. 

4 Huntington, Explorations in Turkestan, Carnegie Institution. 

5 Dana, Manual of Geology, 4th ed., p. 970, and Upham, Am. Geol., Vol. VI, 
p. 327, and Am. Jour. Sci., Vol. XII, p. 33. 


CAUSE OF GLACIAL CLIMATE 646 


and popular form, the hypothesis would seem to require a great 
elevation of a large part of two continents for each ice epoch, and a 
great depression for each interglacial epoch, an extremely improb- 
able sequence of events. This hypothesis has lost rather than 
gained favor, as evidence has accumulated. 

Astronomic hypotheses. An ingenious semi-astronomical hy- 
pothesis was advanced by Croll! in the latter part of the last cen- 
tury, and for a time it was widely accepted. It is founded pri- 
marily on variations in the eccentricity of the earth’s orbit, combined 
with the precession of the equinoxes. Plausible as the hypothesis 
seemed at the outset, prolonged study has tended to weaken, rather 
than strengthen it. ; 


The orbit of the earth is slightly elliptical, and this ellipticity is subject to con- 
siderable variation. This does not alter the total amount of heat received from 
the sun by the earth, or by either hemisphere; but it affects the distribution of heat 
within the year, shortening or lengthening the cooler and warmer seasons, according 
as they fall in the perihelion or the aphelion part of the earth’s orbit. Thus the 
hemisphere which has summer in perihelion has a short summer with much heat 
per hour; the other hemisphere has a long summer with less heat per hour. The 
precession of the equinoxes reverses the seasonal relations of the hemispheres every 
10,500 years. At present the earth is nearest the sun in winter in the northern 
hemisphere (summer in the southern hemisphere). In 10,500 years (owing to the 
precession of the equinoxes) the earth will be nearest the sun in the summer of the 
northern hemisphere (winter of the southern hemisphere). We shall then have a 
shorter summer with more solar heat per hour than now, and a longer winter with 
less heat per hour. Croll’s hypothesis is built upon the belief that snow-accumula- 
tion would be favored by long winters, and snow-melting reduced by short summers. 
The hypothesis is that the glacial epochs occurred during the period of aphelion 
winters in times of great eccentricity. 

It is admitted that these astronomical relations are insufficient in themselves 
to produce the observed glaciation, and so certain terrestrial conditions were made 
important elements in the working force of the hypothesis. It was held that the 
zone of the trade-winds and the thermal equator would be shifted from the glaciated 
hemisphere toward the warmer one, and that this shifting would turn a large 
part of the warm equatorial waters away from the cooler hemisphere. Croll held 
that if the trade-wind belts were shifted southward a few degrees, a large part of 
the equatorial current would be south of Cape St. Roque, and so turned into the 
South Atlantic, greatly lowering the temperature of the northern hemisphere. 
When the southern hemisphere was passing through its cold period, nearly all the 
equatorial current would be north of St. Roque, and this would give the northern 
hemisphere a moist interglacial epoch. 

If the hypothesis were correct, (1) glacial epochs should alternate between the 
northern and the southern hemispheres, and (2) their duration should be iimited 


1 Climate and Time in their Geological Relations; James Croll, pp. 312-328; 
Climate and Cosmology and The Cause of the Ice Age, Sir Robt. Ball, 


650 THE PLEISTOCENE PERIOD 


to an appropriate fraction of the precessional period. This appropriate fraction is 
probably about that which effective winter bears to the whole year. In the middle 
latitudes, the effective period of cold would perhaps be 5,000 or 6,000 years. These 
features of the hypothesis afford a means of testing it. If it be true, the glacial 
epochs should be of equal length; all of them should be short, and all of those in the 
same period of eccentricity, equally distant from each other in time. If the com- 
puted periods of eccentricity are correct, there could be only a few alternations of 
glaciation between the hemispheres within a given period of high eccentricity, 
and none of them could be more recent than 60,000 years. Croll placed the close 
of the glacial period 80,000 years ago. 

The glacial studies of recent years seem to show that the intervals between the 
different invasions are of very unequal duration, and that the most recent is 
relatively young. It has also been found that glaciation was extended notably 
beyond its present limits on the lofty mountains of the equatorial regions, though 
climate there should not have been much affected. The Labradorean and Kee- 
watin ice-sheets pushed out from what appear to have been their centers about 1,600 
and 1,500 miles respectively. If one foot per day be allowed for the advance of 
the margin — an estimate much beyond the probabilities — it would take more 
than 20,000 years for the ice-edge to reach the extension observed. This is almost 
the whole of a precessional period. Nor is the difficulty escaped by assuming 
that the snow-field grew up simultaneously over the whole area, or some large part 
of it, for bowlders are found 600 to 1,000 miles from their probable sources. To 
allow time for the residue of winter snow above summer melting to build itself up 
to a height capable of giving effective motion, and then to allow time to carry 
drift this great distance at any probable rate of motion, taxes the hypothesis very 
severely, to say the least. 

Other astronomical hypotheses. Attempts have been made to base other theories 
on the eccentricity of the earth’s orbit, and also on variations in the obliquity of 
the ecliptic; but none of them has gained much acceptance. ‘They encounter most 
of the difficulties of the Crollian hypothesis, in somewhat different forms. There 
have been speculations upon the possible passage of the earth through cold regions 
of space, but there is no astronomical basis for them. 

It was early suggested that the axis of the earth may have been shifting its 
geographic position, and that the Pleistocene glaciations were but polar glacia- 
tions of the existing type, at a time when the north pole was 15° or 20° south of its 
present position. So long as the theory of a thin crust resting on a liquid nucleus, 
and capable of sliding over it, was accepted, the mechanical difficulties of this 
hypothesis did not seem insuperable; but if the earth is essentially rigid, as now 
seems certain, the dynamic objections to this hypothesis are fatal. 


Atmospheric hypotheses. The leading hypothesis of the atmos- 
pheric class is based chiefly on a postulated variation in the constitu- 
tion of the atmosphere, especially in its amount of carbon dioxide 
and water. Both these elements have high capacities for absorbing 
heat, and both are being supplied constantly and constantly con- 
sumed. Periods of great land elevation and extension are periods 
of great erosion and of great consumption of carbon dioxide, for 
under these conditions weathering is at a maximum, and carbon 


CAUSE OF GLACIAL CLIMATE 651 


dioxide from the air takes part in the decomposition of rock in a 
large way (p. 264). So also, at times of great land elevation and 
extension, the sum total of evaporation of water is reduced, and the 
average amount of water vapor in the air is correspondingly lowered. 
The great elevation of land at the close of the Tertiary seems to 
afford conditions favorable both for the consumption of carbon 
dioxide in large quantities, and for the reduction of the water con- 
tent of the air. Depletion of these heat absorbing elements was 
equivalent to the thinning of the thermal blanket which they con- 
stitute. If it was thinned, the temperature was reduced, and this 
would further decrease the amount of water vapor held in the air. 
The effect would thus be cumulative. The elevation and extension 
of the land would also produce its own effects on the prevailing 
winds and in other ways, so that some of the features of the hypso- 
metric hypothesis form a part of the atmospheric hypothesis. This 
hypothesis also takes into account the action of the ocean in ab- 
sorbing and giving forth carbon dioxide under the varying condi- 
tions that prevailed. It is thus a highly complex hypothesis and 
cannot be set forth in detail here.! By variations in the consump- 
tion of carbon dioxide, especially in its absorption and escape from 
the ocean, the hypothesis attempts to explain the periodicity of 
glaciation. 

While this hypothesis is still new and on trial, it is the only one 
which has been worked out into such detail as to fit the leading facts 
now developed by studies of the glacial formations. It should be 
understood, however, that its truth remains to be established, and 
that modifications and additions may yet be required. 


Hypotheses have been based on the direction of the prevailing winds and also 
upon the degree of cloudiness; but these have not been satisfactorily connected 
with known causes and with the conditions prevailing in Pleistocene times. Fur- 
thermore, they have not been shown to fit the facts of periodicity and localization, 
facts which all hypotheses must meet before they can have serious claims to accept- 
ance. 


FORMATIONS OUTSIDE THE ICE-SHEETS 


While the glaciation of middle and high latitudes was the most 
striking event of the Quaternary period, by far the larger part of 
the earth’s surface was not affected directly by the ice, and outside 
the area of the continental ice-sheet, the commoner phases of erosion 


1For a fuller exposition of this hypothesis see Chamberlin and Salisbury’s 
Earth History, Vol, III, pp. 432-446, 


652 THE PLEISTOCENE PERIOD 


and deposition were in progress, and non-glacial Pleistocene forma- 
tions are widespread. Under the varied conditions of the period, 
various classes of deposits were made, among which were the fol- 
lowing: (1) Eolian deposits, conspicuous along many shores and 
rivers, and in sundry arid regions, and inconspicuous as dust over 
much larger areas. (2) Fluviatile deposits, made by streams (a) 
with, and (b) without, connection with the ice. These deposits occur 
along most streams of low gradient, and along many others. Kin- 
dred deposits were made by sheet-floods and temporary streams, ~ 
even far from the courses of permanent streams. (3) Lacustrine 
deposits of both the glacial and non-glacial types, made in existing 
lakes and about their borders, and also over the sites of the numerous 
lakes which have become extinct since the beginning of the period. 
(4) Deposits made by springs. (5) Terrestrial organic deposits 
(peat, calcareous marl, etc.) occur outside the area directly affected 
by the ice, but are more common in the ponds and marshes to which 
glaciation gave rise. (6) Marine deposits, on lands submerged 
during the Pleistocene period, and doubtless over essentially all of 
the ocean bottom. (7) Volcanic rocks of Pleistocene age are found 
in our continent, chiefly west of the Rocky Mountains, though 
volcanic dust is distributed widely on the Great Plains. All these 
kinds of deposits were doubtless made at other periods, but have 
not been preserved so generally. 

The average thickness of the Pleistocene deposits is not great. 
Pleistocene accumulations of debris at the bases of mountains are 
several hundreds of feet thick in some places; but otherwise the 
thickness of non-glacial Pleistocene deposits rarely exceeds a few 
score feet. 

Atlantic and Gulf coasts: Columbia series. On the Coastal Plain 
of the Atlantic and the Gulf of Mexico, there is a widespread but 
thin body of gravel, sand, loam, and clay, referred to the Pleistocene 
period. It ranges from sea-level up to altitudes of several hundred 
feet, though most of it lies below 200 feet. All of the non-glacial 
post-Tertiary deposits of the Atlantic and Gulf plains were formerly 
grouped together under the name Columbia; but the materials 
formerly grouped under this name represent at least three some- 
what distinct stages of deposition. 

The oldest subdivision of the Columbia series (Qc, Fig. 535) is 


1 Reports of the State Geologist of New Jersey, 1897-1900; also Philadelphia 
folio U. S. Geol. Surv. 


NON-GLACIAL FORMATIONS 653 


found at levels higher than those of the younger subdivisions. In 
the principal valleys, it constitutes broad, mostly rude terraces, 
which rise up-stream. Up the Potomac, the Susquehanna, the 


5 


Gp 


ci 


Fig. 535. Diagram showing the relations of the three divisions of the Pleisto- 
cene as seen in valleys. Qc=the high-level Columbia, 0f=the low-level Columbia 
(or Pensauken), and Qcm, the Cape May formation. 


Delaware, and other valleys, the terraces rise to altitudes notably 
above those attained by the formation outside the valleys. In the 
District of Columbia, the second member of the Columbia series 
(Qp) covers rock terraces 100 feet or so below the oldest member 
phase of the series (Fig. 535). The relations of the two subdi- 
visions indicate that extensive erosion followed the deposition of 
the first, and that the broad valleys then developed were subse- 
quently aggraded by sediments similar to those of the preceding 
epoch of deposition. The two deposits are so nearly alike in com- 
position that their separation is based chiefly on their topographic 
relations. ‘The third phase of the composite Columbia is found at 
still lower levels along the streams and coasts. Its disposition is 
such as to show that the second phase of the Columbia formation 
had been extensively eroded before the deposition of the third. In 
the valleys formed during this interval of erosion, and along the 
coast at accordant levels, the third member of the series finds its 
chief development. 

The various members of the Columbia series rest unconformably 
on older formations. On the Atlantic Coast, the oldest division 





Fig. 536. Diagram showing the theoretic relations of the three principal 
subdivisions of the Pleistocene outside the valleys, along a line normal to the coast. 
The letters have the same significance as in Fig. 535. 


rests now on the Lafayette formation, and now on terranes from 
which the Lafayette had been eroded before the deposition of the 
Columbia series (Fig. 537). 

The Columbia series rarely contains fossils; but at a few points 


654 THE PLEISTOCENE PERIOD 


shells of fresh-water mollusks have been found and at a few points 
marine shells,— all within a few feet of sea-level. 

The origin of the Columbia formation presents much the same 
problems as that of the Lafayette, and is probably to be explained 
in much the same way. ‘The series is looked upon as largely sub- » 
aérial (pluvial and fluvial), the result of land aggradation. The 





Fig. 537. Unconformable contact between the Columbia formation and the 
Potomac, Washington, D. C. (Darton, U. S. Geol. Surv.) 


occasion for repeated intervals of deposition on the Coastal Plain, 
separated by epochs of erosion, probably lay partly in changes of 
gradient incident to surface warpings, and partly in the changes of 
climate of the period. (1) Slight further upward bowing of the 
highlands west of the coast probably stimulated the streams descend- 
ing from them to increased erosion, and the deposition of a part of 
their loads on the plain below was a natural result. The poor 
assortment of the material, the common cross-bedding, the numer- 
ous trifiing unconformities, and the absence of fossils, all are con- 
sistent with this interpretation. (2) The second factor contributed 


NON-GLACIAL FORMATIONS 655 


to the same end. The climate of the period was changeable, and at 
least periodically cold, as the recurrent ice-sheets show. Under 
these conditons a larger proportion of the precipitation than now 
was doubtless in the form of snow, and this was favorable to the 
flooding of streams during the melting seasons. Floating ice helped 
to transport the bowlders of the formation, and so to give it the 
heterogeneity which is one of its distinctive features, especially in 
proximity to the glacial drift. The cold climate probably affected 
erosion, and therefore deposition, in another way, for the reduction 
of temperature probably was attended by a reduction of vegetation, 
and this by an increase of erosion. ‘The reduction of vegetation pre- 
sumably was greatest just where erosion was stimulated most readily, 
namely, in the higher altitudes. 

It is conceived, therefore, that the deposition of the principal 
subdivisions of the Quaternary series of the Coastal Plain resulted 
from the combined effect of slight surface warpings and climatic 
changes; that epochs of notable deposition alternated with epochs 
when erosion was dominant in the same regions; and that the ma- 
terials of each principal stage of deposition were deposited, shifted, 
and re-deposited repeatedly. The youngest division of the series 
was essentially contemporaneous with the last glacial epoch, and it 
seems not improbable that the earlier members were deposited 
during earlier glacial epochs. 

In recent times, dunes have been developed at numerous points 
along the coast, and their development and destruction is still in 
progress.!_ Humus deposits also have somewhat extensive develop- 
ment in the tidal marshes, and to a less extent elsewhere. 

Interior. Some of the non-glacial Pleistocene formations of the 
interior, notably the loess, the valley trains, etc., have been referred 
to. Apart from such formations, there are others which seem to be 
measurably or wholly independent of the ice. The widespread 
gravels of the western plains have been referred to (p. 599), but 
their deposition continued through the Pleistocene, and is indeed 
still in progress. There are numerous tracts and belts of dunes 
where conditions favor their development, as in central and western 
Nebraska, and Kansas. Dunes are of common occurrence locally 
even east of the Great Plains, as about the head of Lake Michigan 
and along its eastern shore. Even where dunes are wanting, wind- 
blown sand and dust in small quantities are widespread. 

1 See for example, the Norfolk, Va.-N. C., folio, U. S. Geol. Surv. 


656 THE PLEISTOCENE PERIOD 





LAKE BONNEVILLE 
showing 
(in black) 
THE DISTRIBUTION 


or 


BASALT 


SCALE; | a 





na° nz° ne m-° 





Fig. 538. Map of Lake Bonneville, showing also the areas of basalt (black 
areas), some of which are Quaternary, the lines of recent faulting (full black lines), 
and the deformation of the basin (broken lines). The numbers on the broken lines 
show the height of the Bonneville shore line above the level of the present Great 
Salt Lake, at different places. (Gilbert, U. S. Geol. Surv.) 


NON-GLACIAL FORMATIONS 657 


Outside the region affected by the ice-sheets, erosion rather 
than deposition was the great feature of the Quaternary in the in- 
terior. In the erosion, wind, running water, and ground-water 
have co-operated. 

The West. The Quaternary formations of the west belong to 
all the several categories mentioned on p. 652, and in addition there 
is much glacial drift left by mountain glaciers. Few of these various 
sorts of deposits have received close study over any considerable 
area, though something is known of all. The deposits of some of the 
lakes at various points west of the Rocky Mountains, especially 
those of the Great Basin, deserve special mention. 

Lacustrine deposits. The most considerable of the western 
Pleistocene lakes was Lake Bonneville ' of which Great Salt Lake is 
the diminutive descendant. Its basin is believed to have been due 
to deformation and faulting. Previous to the formation of the lake, 
the basin is thought to have been arid. During the period of 
aridity, such quantities of debris came down from the surrounding 
mountains as to bury their bases to depths of perhaps 2,000 feet at 
a maximum. 

Later, climatic conditions were such as to bring a large lake 
into existence, but after a time it appears to have dried up, probably 
because of another change of climate. Still later, the lake was 
restored, and its water rose higher than before, and found an outlet 
northward. In the course of time, evaporation from the lake 
again became greater than precipitation and inflow, and the lake 
gradually shrank until it became Great Salt Lake. Atits maximum, 
Lake Bonneville was more than 1,000 feet deep, and had an area 
of more than 19,000 square miles; the maximum depth of Great Salt 
Lake is less than 50 feet (average less than 20),and its area but about 
one-tenth that of its ancestor. 

Terraces, deltas, and embankments of other sorts were developed 
about the shores of Lake Bonneville wherever the appropriate con- 
ditions existed (Figs. 202 and 539), and because of the aridity of 
the climate since the lake sank below them, they have been modified 
but little by erosion. As the lake dried up, deposits of salts were 
made, among which sodium chloride and sodium sulphate are most 
abundant. Great Salt Lake is estimated to contain 400,000,000 
tons of common salt, and 30,000,000 tons of sodium sulphate. 

Igneous eruptions (Fig. 538) have taken place in the basin at 

1 Gilbert, Mono. I, U. S. Geol. Surv. 


658 THE PLEISTOCENE PERIOD 


various stages of the lake’s history, and even since Lake Bonne- 
ville disappeared. Since this time, too, there has been faulting in 
the basin, with displacements of as much as 40 feet (Figs. 538 and 
541). Furthermore, the shore lines of the former lake have been 
warped so that some parts are more than 300 feet higher than others 
(Fig. 538). 

Farther west, but still in the area of the Great Basin, were other 
lakes, probably contemporaneous with Bonneville. Among them 

















ae. 
Fig. 539. Shore of former Lake Bonneville, Wellsville, Utah. (U.S. Geol. Surv.) 


Lake Lahontan ! was of importance. Its history and that of a lake 
which occupied a part of Mono Valley, California, were similar to 
that of Lake Bonneville. 

Glacial effects. The extent of glaciation in the western moun- 
tains was outlined in the early part of this chapter. The erosive 
work of the mountain glaciers was considerable, as shown both 
by the extensive deposits of glacial drift, and by the forms of the 
valleys which the glaciers occupied. The most massive accumula- 
tions of drift are in the form of lateral moraines, which in some 
cases are nearly or quite 1,000 feet high. Under the conditions of 
active drainage which existed in the mountains, much of the glacial 


‘ Russell, Mono. XI, U. S. Geol. Surv. 


NON-GLACIAL FORMATIONS 659 


debris was carried beyond the ice by the water flowing from it, 
and deposited in the valleys and ‘‘parks,” or on the plains below. 
Glacial cirques, the result of a peculiar phase of glacier erosion, are 





Fig. 540. Faulting on the shore of Lake Bonneville. (Church.) 


well developed in many of the glaciated valleys as, for example, in 
the Uinta Mountains. 

The characteristics of mountain valleys which were occupied 
by considerable glaciers are essentially constant. They include 
(x) well developed cirques at the heads (Pl. XIII); (2) the upper 
parts of the valleys were so thoroughly cleaned out by the ice that 
little loose debris, except that due to post-glacial weathering, re- 
mains; (3) numerous tributary valleys are hanging (Fig. 151), and 
their waters form cataracts; (4) at and near the limits of the ice, 


Fig. 541. Fault scarps in the moraine at the mouth of the Little Cottonwood 
Canyon, Wasatch Mountains. (Gilbert, U. S. Geol. Surv.) 


at stages when its end or edges remained for a time nearly constant 
in position, there are heavy accumulations of drift, lateral moraines 
being as a rule more conspicuous than terminal; (5) the valleys 


660 THE PLEISTOCENE PERIOD 


contain lakes (Pl. XIII), some of which occupy rock basins, and 
some basins produced by drift dams; and (6) valley trains or out- 
wash plains below the moraines. The partial removal of these 
deposits has developed terraces (Fig. 124). 

Glacial lake deposits. By obstructing valleys, the mountain 
glaciers of the west gave rise to numerous temporary lakes in which 
lacustrine sediments were laid down. ‘The extent of such lakes in 
the west and northwest has not been determined, but where glacia- 
tion was extensive, derangement of the drainage was common, and 
deposits of glacio-lacustrine clay, hundreds of feet deep, are known 
at some points. Where such deposits were made in narrow valleys 
now drained, they have been removed in part, and their remnants 
constitute terraces. 

Alluvial and talus deposits. In the basin region of Utah and 
Nevada there are exceptional deposits of detritus, the accumulation 
of which was favored by topography and climate. The mountain 
ranges of the basin region are separated by broad depressions. 
From the steep slopes, detritus is carried down both by descending 
torrents and by gravity, and while it is largely deposited at and 
against the bases of the mountains, some of it is spread widely over 
the surrounding plains. This debris is mainly unstratified, or poorly 
stratified, and some of it is very coarse. It appears in greatest 
quantity where canyons issue from the mountains, and in such situa- 
tions there are huge fans of bowlders, some of them 1,000 feet in 
height. The torrents were able to carry this coarse material so 
long as they were confined within the canyons, but with the change 
of gradient below, the water gave up its load. As the glacial de- 
posits increase in importance to the north, talus and other sub- 
aérial accumulations become less conspicuous, and are much less 
considerable in Montana, Idaho, and Washington than in the more 
arid and unglaciated regions farther south. 

Eolian deposits. The wind is an important agent of erosion 
and deposition in the west. Its erosive work is shown in the pecu- 
liar carving which affects the cliffs and projections of rock at many 
points (Fig. 14), and its depositional work by the dunes, which 
are not rare. ‘The erosive work of the wind here is far greater than 
is commonly appreciated by those unfamiliar with arid regions. 

Deposition from solution. About many springs, as in the Yel- - 
lowstone Park, deposits of siliceous sinter and calcareous tufa are 
now making (Fig. 31). Considerable deposits of a similar nature 


PLEISTOCENE CHANGES OF LEVEL 661 


antedate the present by a notable interval of time, but probably 
fall within the limits of the Quaternary period. 

Marine deposits. At some points along the western coast of the 
United States marine deposits extend inland some distance from 
the sea. They reach altitudes of 200 or 300 feet in California } 
and Oregon, and perhaps even higher. ‘The submergence indicated 
by the position of these beds must have given origin to considerable 
bays in the lower courses of the Columbia and Willamette valleys. 
By far the larger part of the marine Quaternary deposits of the 
coasts of the continent are still beneath the sea. 

Igneous rocks. The Quaternary eruptions of North America 
_ have not been separated clearly from those of the late Tertiary, 
but there are some igneous rocks which are Quaternary, some of 
them even late Quaternary. Mount Shasta shows several post- 
glacial lava-flows, and there are small cinder cones on alluvial 
cones at the east base of the Sierras in southeastern California. In 
southern California (Mohave Desert) and northern Arizona (vicinity 
of Flagstaff) there are cinder cones and lava-flows of limited extent 
which are so slightly touched by erosion that there can be little 
doubt that they date from a time long subsequent to the beginning 
of the Quaternary period. Judged by the same criteria, there are 
lava-flows and cinder cones of Quaternary age in New Mexico, 
Colorado, Utah, Nevada, Oregon (p. 230), Idaho, Washington, 
and at various points in the Sierras.” On many of them vegetation 
has hardly begun to gain a foothold. Gilbert estimates that of 
250 lava-fields observed in these states, 15% are of Pleistocene age, 
and of 350 volcanic cones in the same states, 60% are considered 
to. be Pleistocene.* Volcanic ash is interbedded with loess at vari- 
ous points in eastern Washington and Oregon,‘ and overlies glacial 
moraines in some parts of Alaska. 


CHANGES OF LEVEL DURING THE PLEISTOCENE 
The very considerable changes of level which marked the closing 
stages of the Pliocene have been mentioned, and many of them 
doubtless continued into the Pleistocene. Minor movements of 
later date, such as those which affected the basins of Lakes Bonne- 
ville and Lahontan during the Pleistocene also have been noted. 


1 Ashley, Jour. Geol., Vol. III, pp. 446-450. 

2 See published folios of Washington, Oregon, California, and Idaho. 
3 Mono. I, U.S. Geol. Surv., pp. 323-337. 

4 Jour. Geol., Vol. IX, p. 730. 


662 THE PLEISTOCENE PERIOD 


Such changes are probably but a meager index of the crustal warp- 
ings of the period. Specific data on this point are less abundant 
than could be desired, for the phenomena of erosion and deposition 
which followed the elevation at the close of the Tertiary are not 
readily differentiated from similar phenomena resulting from later 
elevations. Nevertheless, evidence of Pleistocene changes of level, 
as distinct from late Pliocene, are not wanting, especially near the 
coasts and about the shores of the Great Lakes. 

From the evidence at hand, it appears that deformative move- 
ments were widespread both in the western mountains and in the 
area covered by the great ice-sheets. In general, the areas covered 
by the ice-sheets have risen since the ice melted. It is a tenable © 
hypothesis that the rise, or some part of it, resulted from the melting 
of the ice, and that it followed a depression caused by the weight 
of the ice. The rise of the land has been greatest, on the average, 
where the ice was thickest. This rise of the glacial centers is shown 
in various ways, but especially by the raised beaches along the 
coasts, and by the deformed shore lines of the interior lakes. Thus 
the shore lines of Lake Agassiz are considerably higher at the north 
than at the south, their inclination being as much as a foot to the 
mile in the northern part of the basin. The shore lines of Lake 
Iroquois (p. 639) decline from the northeast to the southwest at 
the average rate of three and a half feet per mile. The beaches of 
Lake Algonquin (Fig. 530) are 25 feet above the present lake at 
Port Huron, and 635 feet above the lake at North Bay, Ontario. 
The shore lines of the other lakes show comparable warping. 

There have been changes of level, though less extensive in 
most places, in regions which were not glaciated. Thus along the 
Atlantic coast south of the drift there have perhaps been complex 
movements, but of no great range, in the course of the period. On 
the whole, elevation (relative) appears to have exceeded depression, 
but the latest movement (present) appears to have been one of sink- 
ing, as the drowned ends of the valleys show. 

It is not improbable that movements of equal magnitude have 
affected the interior regions of the continent, but, except about 
the lakes, there is no datum plane like the sea-level to which these 
changes may be readily referred. Ina few places, local deformation 
is notable. In New York and Ohio, the solution of underlying gyp- 
sum and salt is suspected of being the occasion of some of the slight 
deformations observed. 


LIFE 663 


Some of the islands of Southern California seem to have risen, 
relatively, some 1,500 feet since the Pliocene. Other parts of the 
California coast, and some of the adjacent islands, have been sinking 
during the same period.! Near San Francisco, the surface is thought 
to have ranged from 1,800 feet below its present level to 400 feet 
above. Along the northwestern coast of Oregon, a rise of at least 
200 feet during the Pleistocene ? has been estimated. 


Foreign 


The salient points in the glacial history of Europe have been 
sketched, and some indication has been given of the extent of the 
deployment of ice in other continents. It need only be added here 
that outside the areas affected by the ice, there are, in all continents, 
accumulations of sediment of the sorts just enumerated. In 
Europe there are cave deposits of Quaternary, perhaps of glacial, 
age, which are of interest because they contain human relics, prob- 
ably the oldest known. ‘The relics consist of rude stone implements, 
bones of mammals with human markings on them, and bones of 
human beings. 

LIFE 


Destructive effects of glaciation. We must believe that the suc- 
cessive ice-sheets, several million square miles in extent, destroyed 
much life, and caused great changes in that which survived; yet, so 
far asthe record shows, the difference between preglacial life and post- 
glacial life is less than might have been expected. More than half 
the known species of marine Pliocene invertebrates are still living, 
though in the transition between several of the more ancient periods, 
nearly all species disappeared. Of Pliocene plant species, too, many 
are still living; but the land vertebrates of that period were very 
generally replaced by new species, and the same appears to be true 
of the insects. 

When the ice was most extensive, the sum total of life on the 
earth must have been reduced greatly. Even the life of to-day is 
probably less in amount than that of the middle Tertiary. Not only 
this, but existing life is probably but poorly adjusted to its surround- 
ings, for it is improbable that, in the millions of square miles where 
life was destroyed by the ice, there has yet been. worked out the 

1 Lawson, Bull. Dept. Geol., Univ. of Calif., Vol. I. Reviewed in Jour. Geol., 


Vol. II, p. 235. 
2 Diller, 17th Ann. Rept., U. S. Geol. Surv., Pt. I. 


664 THE PLEISTOCENE PERIOD 


best balance (1) between the vegetation and the soils and climate on 
which it depends, (2) between plants and herbivorous animals, and 
(3) between the carnivorous animals and the herbivores on which 
they prey. 

To-and-fro migration. An important biological effect of the 
ice-sheets on life, was forced migration. With every advance of 
the ice, the whole fauna and flora of the region affected had to move 
on in front of it, or die. The arctic species along the ice border 
crowded upon the sub-arctic forms just south of them, these in turn 
crowded upon the cold-temperate species beyond, and so on. It 
is not unlikely that even the tropical zones were somewhat narrowed. 
During the interglacial epochs, migrations were reversed. As the 
advances and retreats of the ice caused migrations back and forth, 
every organism was obliged to adapt itself to a new zone, to migrate, 
or to die. There appear to have been four or five such to-and-fro 
migrations in America and Europe, and the extent of the migrations 
was several hundred miles, and in some cases perhaps one to two 
thousand miles. During some of the interglacial epochs, the life 
of middle latitudes indicates a climate milder than the present, and 
this implies that the ice-sheets were reduced at least as much as 
now. During some of the interglacial epochs, northern lands seem 
to have supported as many plants and animals as now. Geological 
evidence warrants the belief that at least some of the interglacial 
intervals were long enough, and their climates warm enough, to per- 
mit a complete northward return of the life which was forced south 
during glacial epochs. 

Relics of glacial migrations. Significant evidence of the to-and- 
fro migrations of the period is found in the life of the higher moun- 
tains within or near the borders of the once glaciated areas. When 
the ice was near these mountains, arctic life only could have existed 
there. As the ice retired to the north, the arctic life of the surround- 
ing lowlands moved northward also, and life from the temperate 
zone came on to take its place; but in the mountains the arctic life 
still found congenial conditions by moving up to higher and higher 
levels as the climate became warmer. In this way arctic life be- 
came isolated in the high mountains. Plants, insects, and small 
mammals whose kin now live in the arctic zone, remain to this day 
in some of the higher parts of the northern Appalachians, and the 
same point is still more strikingly illustrated in the Alps. 

Life of interglacial epochs. By far the larger part of the fossils 


LIFE | 665 


whose exact relations to the ice invasions can be fixed are found in 
the interglacial beds. Of these, the most instructive which have 
been studied carefully in America are those on the Don River and in 
the Scarboro cliffs, near Toronto.1. The fossil-bearing beds are 
underlain by a sheet of bowlder clay older than the late Wisconsin 
sheet of drift. The upper surface of this underlying till was eroded 
before the overlying fossiliferous interglacial beds of stratified sand 
and clay were deposited upon it. After the erosion of the latter, a 
thick body of drift of Late Wisconsin age was deposited upon it. 

The lower part of these interglacial beds contains fossils of a 
warm-temperate fauna and flora, while the upper contains the relics 
of a cold-temperate fauna and flora. Up to 1900, the lower beds 
had yielded 38 species of plants, many of which indicate a climate 
appreciably warmer (3° to 5°) than that of the same region now. 
Among these are the pawpaw and the osage orange, which now 
flourish farther south. The fauna includes about 40 species of 
mollusks, some of which are now living in Lake Ontario, some in 
Lake Erie, while some are not known in the waters of the St. Law- 
rence system. The fossils of the upper beds include 14 species of 
plants, and 78 species of animals, mostly beetles. This assemblage 
implies a climate of about the type which now prevails in southern 
Labrador. ‘The arctic fauna and flora which should have followed 
this cold-temperate one, marking the approach of the next ice-sheet, 
are undiscovered. 

In other interglacial formations there is evidence at many 
points of an ample growth of vegetation, recorded in peat and muck 
beds, in humus-bearing soils, and in twigs, limbs, trunks, etc., of 
trees, but from them few species have been identified. Recently, 
bones of horses (more than one species) have been found in the 
Aftonian interglacial beds in Iowa,’ along with bones of elephants 
and mastodons. 


Marine Life 


On northerly coasts. During that stage of the Wisconsin 
glaciation when the eskers of Maine were being formed, and the sea- 
level stood higher than now relative to the land along that part of 

1 Coleman, Interglacial Fossils from the Don Valley, Toronto, Am. Geol., 
Vol. XII, 1894, pp. 86-95, with references to earlier literature; also Glacial and 
Interglacial Beds near Toronto, Jour. Geol., Vol. IX, 1901, pp. 285-310. Professor 


Coleman thinks (1913) that the Don beds. are of Aftonian age. 
2 Calvin, G. S. A., Vol. XX. $e aie 


666 THE PLEISTOCENE PERIOD 


the coast, arctic mollusks lived along the shore and were buried in 
marine clays deposited while the eskers were being made.! The 
same species live now in waters that are near the freezing point most 
of the year. Remains of walruses, seals, and whales also have been 
found. When an arm of the sea occupied the lower St. Lawrence 
and Champlain valleys (p. 640), it was peopled by a marine fauna 
similar to that which now lives about the mouth of the St. Lawrence 
and on the coast of Labrador. 

On southerly coasts. Away from the immediate influences of 
the ice-sheets, the record of marine life does not indicate any pro- 
found departure from the progressive modernization that had been 
in progress through the Tertiary period. It has been stated by Dall 
that the Pleistocene fauna of the Atlantic coast does not imply as 
cold waters as the Oligocene fauna does, and by Arnold that the 
Pleistocene fauna of the California coast does not indicate a climate 
as cool as that of the Pliocene. It is to be noted, however, that the 
known marine record may not cover more than a small part of the 
Pleistocene period, and it is not certain — perhaps not probable — 
that the portion represented corresponds to any one of the glacial 
epochs. When the ice was pushing into the ocean on the coast of 
Maine, as in the late Wisconsin epoch, and an arctic fauna occupied 
that coast, it is scarcely probable that a warm-temperate fauna 
‘ived on the southern coast; nor is it probable that, when icebergs 
were being discharged into Puget Sound, and along all the coast 
farther north, a warm-temperate fauna lived on the California 
coast; but warm-temperate faunas on those coasts during inter- 
glacial epochs are entirely consistent with a climate such as that 
suggested by the Don River beds. | 


Terrestrial Life of Non-glaciated Regions 


The life of the lands far from the glaciated areas cannot now 
be correlated closely with the glacial and interglacial stages. In 
North America, northerly types such as the mammoth and mastodon, 
the bear, bison, reindeer, and musk-ox, apparently driven south by 
the advancing ice, were characteristic of these faunas. In the mid- 
latitudes of North America there were several types on the verge of 
extinction, such as the horse, tapir, llama, and saber-tooth cat. It 
is not improbable that there was intermigration with Eurasia by 


1 Stone, Mon., U. S. Geol. Surv., XXXIV, 1899, pp. 53-54, and Bastin, Rock- 
land, Me., folio, U. S. Geol. Surv. 


LIFE 667 


the northeastern (Greenland-Iceland) or northwestern (Behring 
Strait) routes during the interglacial epochs. Another prominent 
feature of the land faunas far from the ice was a group of southern 
forms consisting of gigantic sloths, armadillos, and water-hogs, 
whose forebears had come from South America a little earlier, by 
way of the Isthmus of Panama. 

The boreal group. As in the Pliocene, proboscidians dominated 
the fields and forests of middle latitudes. The mammoth ranged 





Fig. 542. An-interpretation of Mastodon americanus by G. M. Gleeson. 
(From painting in National Museum, Washington.) 


from Mexico northward, reaching Canada and Alaska during inter- 
glacial epochs. In Siberia, the mammoth was covered with wool 
and hair, and was obviously adapted to a cold climate. The mam- 
moth survived the glacial period in America, and its tusks and 
skeletons are found in beds of peat and muck which have accumu- 
lated since, in northern United States and Canada. The mastodon 
also ranged northward into Canada, but since it emigrated to South 
America and crossed the tropics, it must have been adapted to a 


668 THE PLEISTOCENE PERIOD 


warm Climate also. It likewise outlived the glacial period. Willis- 
ton suggests that while mammoths were abundant on treeless plains, 
mastodons were confined mostly to valleys and forests, notably those 
of the eastern states, the eastern part of the Mississippi basin, and ~ 
the Pacific coast. 

Several species of horses have been found in western beds 
referred to the Pleistocene period. A gigantic elk ranged from 
Mississippi to New York. ‘Two or three species of buffaloes roamed 
over the Ohio valley and southward to the Gulf, and remains of the 
musk-ox and reindeer, distinctively arctic animals, have been found 
as far south as Virginia and Kentucky. 

The southern group. Besides this assemblage of more or less 
boreal forms pushed southward by glacial advances, there was the 
group of South American immigrants, the monster sloths, and a 
gigantic armadillo with a strong carapace and a massive tail p ated 
with spiked ossicles (Fig. 543). The remains of this group. have 
been found chiefly in caverns, in the muck and mire about salt 
springs, and in fluvial deposits, the precise ages of which are difficult 
to fix. In the climate of such an interglacial stage as that which 
permitted pawpaws and osage oranges to flourish about Toronto, 
there was apparently nothing to prevent these animals from ranging 
northward to Pennsylvania and Oregon. 






Life in Eurasia 


The faunas of Europe underwent changes similar to those already 
sketched for America. During the first glacial epoch, an arctic 
fauna lived in the North Sea, while during the first recognized inter- — 
glacial epoch, the arctic fauna retreated northward. At this time 
a flora comparable to that now living in England was found in the 
British Isles, while the hippopotamus, elephant, deer, and other 
mammals invaded Britain by way of the land bridge which then 
connected it with the continent. A similar flora and fauna ad- 
vanced to corresponding latitudes on the mainland. A luxurious 
deciduous flora lived in the valleys of the Alps, and up to heights 
which it no longer attains. ‘Toward the close of this interglacial 
epoch the temperate flora gave place to an arctic flora. 

During the second glacial epoch, according to Geikie,! the ice 
reached its maximum extent in Europe, and arctic-alpine plants 
occupied the low grounds of central Europe, while northern mam- 

! The Great Ice Age, Third Edition, pp. 607-615. . 


LIFE 669 


mals, including the reindeer, the arctic fox, etc., reached the moun- 
tains of southern Europe, and even the shores of the Mediterranean. 
During the second interglacial epoch, a temperate flora and fauna 
succeeded the arctic ones which had just preceded. The plants 
which then occupied northern Germany and central Russia imply a 
climate milder than the present, and the mammalian fauna, which 
included the hippopotamus and elephant (Elephas antiquus), was 
in keeping with the flora. Toward the close of this interglacial 
epoch, northerly forms began to appear, and as the third glacial 





: 2 psd Tinsel en ey al so SE SSE SS SS ae bere eee: HS 


Fig. 543. A club-tailed glyptodon, Dedicurus clavicaudatus, from South 
America. (Lydekker.) 


epoch came on, northern types advanced well to the south. In the 
third interglacial epoch the climate seems to have been congenial to 
a cool-temperate fauna. 

During the remaining epochs the oscillations of the ice appar- 
ently were less. Corresponding to these diminishing oscillations the 
to-and-fro migrations of life appear to have become less extensive. 

Pleistocene life of other continents. While the Pleistocene life 
of North America was similar to that of Europe, that of South 
America had a character quite its own. Its most distinctive fea- 
tures were (1) gigantic sloths and armadillos, indigenous to South 
America and very numerous, and (2) descendants of the Pliocene 
mammals which had migrated from North America. Among the 
northern immigrants were horses, mastodons, llamas, tapirs, wolves, 
and a variety of rodents, 


670 THE PLEISTOCENE PERIOD 


Owing to the isolation of Australia, its life was peculiar to itself. 
The vertebrate fauna consisted of marsupials and monotremes 
exclusively. In general, they differed specifically from those now 
living, and were, on the whole, larger. Although glaciers had but 
slight development in Australia, the effects of the widespread 
refrigeration of the higher latitudes was doubtless felt. Compara- 
tively little is known of the Pleistocene life of Africa, but a mod- 
erate climate in the northern portion seems to be indicated. 


Man in the Glacial Period} 

In America. Previous to the last decade of the last century, 
much prehistoric material of human origin had been collected and 
widely accepted as proof of man’s presence 
in America in glacial times; but later studies 
have disclosed. weaknesses both in the evi- 
dence and in its interpretation. ‘The result 
is that man’s antiquity in America is a more 
open question to-day than it was thought to 
be twenty years ago. 

These prehistoric human relics in America 
range from the rudest stone chips and flakes 
to skillfully fashioned and polished handi- 
work in stone, metal, and bone. Following 
European precedent, the rougher artefacs ? 
were Classed as paleolithic, and interpreted as 
indicating the presence of Paleolithic man 

Fy (and of the Paleolithic or Old Stone age) in 
Sones be ee America. The more perfectly fashioned 
Cavern, Torquay, Eng- artefacs were classed as neolithic, with cor- 
land, seen on the face yesponding reference to the Neolithic (New 
and edge. (Evans.) 

Stone) age. Some students properly regard 
‘‘paleolithic”’ and ‘‘neolithic” as stages of early art, not as chrono- 
logical ‘‘ages,”’ or geologic divisions, though the terms have been 
much used in the latter sense. 

The relics interpreted as paleoliths consist chiefly of rudely 
chipped pieces of flint, quartz, argillite, etc. (Fig. 544). The neoliths - 

1 See references, p. 676. 

* The term ‘“‘artefac’”’ designates any object fashioned by man, in any way 
or for any purpose, or, incidentally, without purpose. It includes stone chips, 


broken and rejected material, and various forms of by-products, as well as imple- 
ments, weapons, ornaments, etc. 










Zeal 


Sz 







= se 


hy 





A 





LIFE 671 


include a wider range of stone artefacs, typified by well-chipped 
arrow-points, spear-heads, knives, and scrapers of flint or quartz, 
and by the ground and polished axes, chisels, pestles, mortars, and 
other implements of greenstone and similar tough or workable 
rock. The paleoliths, as defined above, were interpreted as the 
work of an earlier and less cultured people, while the neoliths were 
known to have been the implements and weapons of the natives of 
the continent when first invaded by Europeans. It is to be noted 
that the phase of the stone art designated ‘‘neolithic”’ was dominant 
on the continent until recent times, and is scarcely yet extinct. 
Holmes! has shown that the early inhabitants of the country, 
like the later Indians, went habitually to gravel-beds and to out- 





Fig. 545. A series of forms illustrating progressive steps in the manufacture 
of arrow-points from quartz pebbles obtained mainly from shops and village-sites, 
near Anacostia, D. C. (Holmes.) 


crops of appropriate rock to procure the raw material for their stone 
artefacs, and that it was their custom to test and to rough-out the 
material on the ground, leaving the chips and rejected material 
scattered about when the rough work was done. The more delicate 
work of shaping the rough material into implements was apparently 
done as need required, at their villages or at other convenient places 

1 Holmes, W. H., A Stone Implement Workshop, Am. Anthropologist, Vol. 
III, 1890, pp. 1-26; Review of the Evidence Relative to Auriferous Gravel Man in 
California, Smith. Rept. 1900, pp. 417-472; Stone Implements of the Potomac- 
Chesapeake Tidewater, Ann, Rept. Bureau of Eth., 1893-94, pp. 1-152, and Jour. 
Geol., Vol. I. 


672 THE PLEISTOCENE PERIOD 


to which stone from the quarries was carried. The stages of manu- 
tacture, as thus interpreted, are shown in Fig. 545. 
Because of this separation of the process of manufacture into 





Fig. 546. A group of figures of chipped-stone artefacs, one of which has been 
regarded as a typical paleolithic implement, front and side view. The rest were 
obtained, in three cases, from modern flint-shops of the region in which the supposed 
paleolith was found, while the fourth was traceable directly to the same shops. 
The discrimination between the paleolith and the rejects is left to the reader. 
(Holmes.) 


two parts, (1) roughing out at the quarries, gravel-beds, etc., and 
(2) shaping tools at dwelling sites or elsewhere, there arose a geo- 
graphic separation of the products. The rude failures and rejects, 
together with the extemporized hammer-stones, cores, flakings, and 


LIFE 673 


chips, were scattered about the sites of the raw material, while the 
completed implements are liable to be found only about the dwelling 
sites, or where, in their use, they were lost or thrown aside. In the 
light of this definite separation, it is not difficult to see how the idea 
of two stages of art arose, and how easily the finds might be mis- 
interpreted. | 

The most available sites for finding suitable raw material in a 
convenient form were river gravels and terrace formations. This 
was especially true in and about the glaciated regions where glacial 
gravels abounded. In them, quartz, flint, chert, etc., were usually 
abundant, in the convenient form of pebbles and cobbles. 

Many of the rude artefacs in question (“‘paleoliths”’) have been 
found chiefly in such gravels, and it was this which cuased them to 
be interpreted as proving the existence of glacial man. Most of the 
artefacs in valley gravels are in their superficial portions, in their 
talus slopes, or in secondary deposits, many of which are of recent 
origin. Of the less superficial finds, many have been shown to be 
cases of relatively recent burial by natural means. The processes 
of streams in cutting down their channels in valley gravels are such 
that superficial material may be buried to very considerable depths, 
as illustrated in Figs. 547-549. The material which was in the top 
originally, may get into the base of the talus, and be buried deeply. 
Similar secondary burial takes place in all sorts of loose material of 
eolian, pluvial, and fluvial origin; and it is to be noted that this is a 
normal process, not an exceptional one. ‘There are other ways, too, 
notably scour and fill (p. 112), in which human relics may be buried 
in river gravels. 

Without further details, it may be said that human relics have 
not been found, in America, in gravels known to have been deposited 
in the glacial period, or before. All that have been reported from 
glacial gravels have been found either in such positions as to show 
that they were buried in post-glacial time, or in such positions as 
to make this inference probable. The existence of man in America 
in the glacial period or before is therefore not demonstrated. 

In Europe. The European data indicating great antiquity of 
man are better than the American. In Europe there are numerous 
caves in which the relics of man, mingled with those of extinct 
animals, have been securely protected by layers of stalagmite-. 
While the ages of the stalagmite layers have rarely been fixed 
with certainty, or well correlated with the glacial stages, they bear 


674 THE PLEISTOCENE PERIOD 


inherent evidence of considerable antiquity. The European cave 
evidence seems to have no strict counterpart in America. 

The association of man with extinct animals is a phenomenon 
that may mean the extension of man’s presence backward, or the 
extension of the animals’ presence forward; and to this double- 
faced problem research has not yet furnished a final key. Obviously, 


2 wocegeccored 


1 River 





Fig. 547. A gravel bluff formed by the undercutting of the adjacent river. 
(After Holmes.) 


IR iver 





Fig. 548. The same at an early stage of talus formation. 


=—--= Te we on Stree eee reese ese ease seaey 
wee oe Memcaccoet > 


Eero. Ne 


pone nn nner en 2° 8S 


r 
a 
us 


ecreeeor” 


River 


ae Pe mnoe 


~weeeceererrrs 





Fig. 549. The same at a late stage, when the slope has become nearly stable. 


however, the larger the number of animal types not known to have 
lived this side the last glacial stage whose remains are commingled 
with human relics, the stronger the presumption of man’s presence 
before the close of the glacial period. From this point of view, 
the European case seems to be strong. 

There is one further feature in the European case that is, at 
least, suggestive. Two climatic groups of animals are associated 


LIFE 675 


with the human relics—a subarctic and a subtropical. In the sub- 
arctic group there were reindeer, mammoths, woolly rhinoceroses, 
musk-oxen, and other boreal forms; in the subtropical group, lions, 
leopards, hippopotamuses, hyenas, southern rhinoceroses, and other 
African types. These contrasted groups, as interpreted by James 
Geikie and others, imply migrations of the kind already sketched 
as characteristic of the glacial period. These seem to indicate, 
therefore, that man lived in Europe before the close of the glacial 
period. 

The relics thus associated with extinct animals have been 
assigned to paleolithic man, and to a primitive stage of culture. 
This interpretation is based on the crudeness of the stone artefacs, 





Fig. 550. Etching of an aurochs on a slab of slate, from the bone cave of Les 
Eyzies, Dordogne, France (% size). This sketch may be instructively compared 
with the similar work of the ancient Assyrians and Egyptians. (Prestwick.) 


rather than upon the evidence of a higher order of art which the 
record presents. If, however, the rude stone artefacs can be inter- 
preted as the waste incidental to the making of good stone imple- 
ments, a more favorable judgment of the art of these ancient peoples 
would be reached. Associated with the ruder artefacs (or paleo- 
liths) there are implements of bone, such as needles, awls, harpoons 
or spears with barbs, etc., implying some advance in art; there are 
carvings that show not a little skill, and drawings in which the ele- 
ments of perspective and shading, as well as skill in delineation, are 
indicated (Fig. 550). These seem to imply a higher stage of art 
development than is consistent with the exclusive use of paleolithic 


676 THE PLEISTOCENE PERIOD 


stone implements. On the whole, present evidence seems to justify 
the conclusion of most European archeological geologists that man 
was present in southern and central Europe during the later part of 
the glacial period, and perhaps even early in the period. A recent 
discovery in Switzerland would seem to place the beginning as far 
back as the interglacial’ epoch which may correspond with the Noe 
American Yarmouth (p. 634). 


A few references relative to the antiquity of man: Chamberlin, T. C., Jour. Geol., 
Vol. X; Geikie, James, The Great Ice Age, pp. 616-6090; and Prehistoric Europe, 
pp. 568 et seq.; Gilbert, G. K., Sci. Am. Supp., Vol. XXIII, 1887; Lyell, Sir Charles, 
Antiquity of Man; McGee, W. J., Am. Geol., Vol. XXII, pp. 96-126; Sci., new ser., 
Vol. IX, pp. 104-105; Upham, Warren, Science, Vol. XVI, pp. 355-6; Am. Geol., 
Vols. XXX and XXXI; Whitney, J. D., Am. Jour. Sci., 2d ser., Vol. 43, pp. 265- 
267, 1867; The Airitecnia Gravels of the Sierra Nevada of California, Cambridge, 
1879. 

1 Science, Vol. XXIX, 1909, P. 359. 


. CHAPTER XXX 
THE HUMAN OR PRESENT PERIOD 


The end of the glacial period. The close of the glacial period 
is usually placed at the time when the ice-sheets disappeared from 
the lowlands in the middle latitudes of Europe and North America. 
Notwithstanding this usage, the ice-sheets had not then disappeared 
completely, and have not even now, for about 10% of the recently 
glaciated area of North America (chiefly in Greenland) is still buried 
inice. ‘These relics of the last glacial epoch show that the continent 
has not yet emerged completely from the glacial period. Indeed it 
is not absolutely clear that there may not be another increase of ice 
before the long series of glacial epochs closes, but the probabilities 
seem to be against it. 

It is not wholly clear that the deformative period which began 
in the late Tertiary, and extended through the Pleistocene, is yet 
completed. We are accustomed to regard it so, and perhaps this 
position is justified; but the movements of post-glacial times are not 
to be ignored (p. 661). A recent movement in the region of the 
Great Plains seems to be suggested by certain physiographic 
features. Many phenomena suggest that the western side of the 
Great Plains was lower than now, relatively, until about the close 
of the glacial period. On the western side of the continent there is 
much evidence of recent movement, some of which appears to have 
taken place since the close of the glacial period, as usually defined. 
Similar phenomena are found in other continents. It is not wholly 
clear, therefore, whether the present is to be regarded as a part of 
that period of deformation which had its climax in the Pliocene, or 
whether it is rather the initial stage of a period of quiescence now 
being entered upon. 


FORMATIONS 


The formations which have been making since the end of the 
glacial period are similar to those of that period, except that gla- 
cial drift is now being made in limited areas only. Most marine 


677 


678 THE HUMAN PERIOD 


post-glacial formations remain beneath the sea, and are not avail- 
able for study. The general character of the formations being made 
will be readily inferred from what has been said in earlier chapters. 


LIFE 


In the seas, and on the land in the tropics, the life of the Pleis- 
tocene period appears to have passed by imperceptible gradations 
into that of the present. In the higher latitudes the transition 
was marked by two exceptional features, the re-peopling of the - 
lands depopulated by the ice, and the invasion of the human race. _ 

Re-peopling the glaciated areas. The re-peopling of the north- 
western half of North America by plants and animals after the re- 
treat of the last ice-sheet was a great event of its kind. Certain 
plants that abounded in Europe before the glacial period were 
forced across the Mediterranean, or southeastward into Asia, and 
did not recross the barriers of water and desert when the climate 
of Europe became mild again. No such barrier intervened in North 
America. ‘There was, however, an ill-defined climatic barrier be- — 
tween the arid plain region of the southwest and the humid forest 
region of the southeast. There is abundant evidence that open 
plains and arid climates had developed in the middle latitudes of 
the west by the later part of the Tertiary, and that these have 
persisted, perhaps with brief interruptions, till now. The pre- 
glacial arid tracts of the west seem to have been distributed much as 
now, while the eastern half of the continent was more moist, and 
covered with forests. 

As the floras and faunas of the western mountain region were 
driven south by the ice, they were hemmed in by mountain barriers 
at the sides, and resisted by arid lands in front. As the trend of the 
mountains was mainly north and south, they defined a series of 
meridional tracts which directed the life migrations. 

In the eastern half of the continent, forests and forest-life were 
driven southward in a more unrestrained way, but for the most part 
they kept within the eastern humid tract. 

Following the last ice-retreat, the life of each of these sections 
moved northward, expanding as it went. ‘The arctic or tundra flora 
and fauna that had probably been crowded into a narrow zone 
fringing the ice-sheet, moved northward through about 20°, and 
expanded to a breadth of 600 or 700 miles in the northern part of 
the continent, and occupied the arctic islands not covered by 


DYNASTY OF MAN | 679 


perennial ice and snow. The zone of this arctic flora and fauna 
now lies mostly north of 60°. The subarctic zone of stunted conifers 
moved northward about 12°, and expanded to a width of 400 to 
600 miles. The cold-temperate belt of deciduous and evergreen 
trees moved a less distance, but expanded almost equally, while 
the warm-temperate flora spread over the territory abandoned by 
the last. 

With each of these vegetal zones went the appropriate fauna. 
The musk-ox, whose remains have been found skirting the glaciated 
area in Pennsylvania, West Virginia, Ohio, Kentucky, Oklahoma, 
Missouri, and Jowa,! has since retired to the extreme arctic regions. 
The reindeer, which had a similar distribution about the edge of 
the ice, occupies the barrens of the northern border of the conti- 
nent, while fur-bearing animals distributed themselves through the 
three northerly zones. 

At the south the floras and faunas of the southeast spread west- 
ward but little, but the arid and prairie floras and faunas of the 
southwest spread eastward at the expense of the southeastern group. 
This does not seem to be equally true in the higher latitudes, where 
the trees of the eastern group are distributed far to the northwest. 

The arid and semi-arid floras and faunas of the southwest seem 
to have pushed the more boreal and arboreous forms to the north- 
ward, or forced them to ascend the mountains; but the movement 
was less sweeping and more complicated than that of the east, 
because of topographic interference and the effect of the lingering 
mountain glaciation. 


The Dynasty of Man 


Human dispersal. As yet there is little geologic evidence 
relative to the place of man’s origin, or to the earliest stages of his 
development. Various considerations connected with his physical 
nature and his distribution seem to point to the warm zone of the 
eastern hemisphere, perhaps southern Asia or northern Africa, as 
the place of his appearance. There are some grounds for the in- 
ference that the earliest dévelopments of those qualities that gave 
him dominance were associated with the open tracts of the sub- 
tropical zone, rather than with the forests of the equatorial belt. 
Subsequent history, as vell as the nature of the case, teaches us that 


1. Hay’s Catalogue of Fossil Vertebrates in North America, Bull. 179, U. S. 
Geol. Surv., 1902: A 


680 THE HUMAN PERIOD 


extreme desert conditions and excessive heights are prohibitive, that 
semi-arid conditions of varying and precarious intensities lead to 
nomadic habits, sparse distribution, and limited social and civic 
evolution, while well-watered plains and fertile valleys, under 
congenial skies, invite fixed habitation and the development of 
stable civil and social institutions. Excessive humidity, dense 
forests, and extreme ruggedness of surface tend to limitation and 
repression among primitive peoples. Early in the history of the 
race, it is presumed that a warm climate was more favorable than 
a severe one. From these considerations and from historical 
evidence arises the presumption that the primitive centers of evolu- 
tion of the race were somewhere in the open or diversified parts of 
the warm tract of the largest of the continents. From this, or from 
some analogous tract in that quarter of the globe, there seem to 
have been divergent movements to all habitable lands. 

A basal factor in the early evolution of civilization was the 
productiveness of the soil. The advance from hunting and fishing 
and herding was dependent essentially on agriculture, and was 
therefore influenced largely by the fertility of the soil and suitable 
climatic conditions. Loss of soil-fertility has been one cause which 
has forced the migration of centers of civilization. In lower lati- 
tudes the upland soils are mostly the residue produced by the 
decomposition of the underlying rocks and not removed by erosion. 
With cultivation, wash and wind-drift are accelerated, and unless 
protective measures are employed, as has not been the case usually, 
the soils are carried away, and barrenness succeeds fertility. There 
are areas in the Orient, once well settled, where nothing grows ex- 
cept such plants as find a foothold in the crevices of the rock. In 
some places, soils underlain by sandy subsoils have been washed 
away, leaving barren wastes. Sands from the exposed subsoil have 
then been driven by the wind over adjacent fertile tracts, making 
them barren. The explanation of much of the former richness and 
present poverty of Oriental peoples no doubt lies in this simple proc- 
ess. Impoverishment of soil threatens many peoples to-day, and 
is in process of actual realization. This is one of the fields in which 
conservation is most important. : 

In glaciated lands the soil-factor has a character quite its own. 
1. Near the centers of ice radiation the old soils were worn away, 
and new soils have not developed in equal amount in their stead. 
Reduced fertility is the result. These areas lie chiefly in high 


DYNASTY OF MAN 681 


latitudes where other factors do not favor human development. 
2. In regions of heavy glacial deposition, which fortunately include 
the greater and the more southerly parts of the glaciated area, a 
deep sheet of comminuted rock-material, ready for easy conversion 
into soil by weathering and organic action, covers great plains. 
Furthermore, the drift has a gentle relief that does not favor rapid 
erosion. North of theborder of theglaciated area in North America, 
in a belt 400 or 500 miles wide, the subsoil of glacial flour and old 
soil, glacially mixed, has an average thickness of about 100 feet. A 
similar statement may be made of a large area in north-central 
Europe. The average thickness of the residuary soils of unglaciated 
regions similarly situated is about 5 feet. The twenty-fold provision 
for permanent fertility thus arising from glaciation seems likely to 
be a factor of importance in the localization of the basal industry 
(agriculture) of mankind, and of the phases of civilization that are 
dependent on it. 

With the evolution of the industrial arts, resources which were 
neglected at first have come to play important parts in the distri- 
bution and in the activities of the race, among which are the long 
and growing lists of mineral resources. Chief among these are the 
metallic ores, the fossil fuels, the mineral fertilizers, and the struc- 
tural and ornamental materials of stone and clay. These now 
influence man’s distribution and activities far more than formerly, 
and they are quite certain to be more influential still in the future. 

The distribution and activities of men recently have come to 
be affected by the distribution of the water-power that arose from 
the deformations of the late Tertiary periods, and the stream- 
diversions of the glacial period. With little doubt, such sources of 
power are to play an increasingly large part in human affairs as time 
goes on and the stored fuels are exhausted. 

With the increasing complexity of human activities, the locali- 
zation of the race will more and more depend on combinations of 
resources and conditions; but it is difficult to see the time when 
persistent fertility of the soil, under favorable climatic conditions, 
co-ordinated with great supplies of fuels, ores, and structural ma- 
terials, will not constitute a decisive and controlling advantage. 

Provincialism giving place to cosmopolitanism. The early 
history of human dispersal was marked by pronounced provincial- 
ism. Early peoples were much isolated by distance and by natural 
barriers, and they often interposed artificial barriers against free 


682 THE HUMAN PERIOD 


intercommunication, and hence against the development of a 
common cosmopolitan type. So long as. hunting and fishing were 
the dominant pursuits, a wider and wider dispersion into small 
tribes was a necessary tendency. That such artificial sources of 
provincialism were more effective than natural ones seems to be 
implied by the fact that while physiological differences sufficiently 
marked readily to characterize varieties are numbered by hundreds, 
dialects sufficiently different to prevent free intercourse are num- 
bered by thousands. Provincial sentiment to-day manifests itself 
more conspicuously in language than in most other ways. 

When efficient water-transportation was developed and the 
control of the sea attained, a period of cosmopolitan tendency was 
inaugurated. This has been greatly accelerated in the last few 
decades, supplemented by rapid land-transportation and electric 
communication, and is rapidly involving the whole race in a cosmo- 
politan movement. Almost the whole world is already in daily 
communication, and most races are more or less habitually inter- 
mingling by travel and trade. That this is to become more and 
more habitual until the whole race shall be in constant inter- 
communication, is not to be questioned. There will then have 
been inaugurated the most marked period of cosmopolitanism, in 
all senses of the term, which the world has ever witnessed. What 
all this will ultimately mean for the race we do not venture to 
predict. 

Man as a geological agency. The earlier geologists were in- 
clined to regard man’s agency in geological progress as rather 
trivial, perhaps because physiographic geology, in which his influ- 
ence is felt chiefly, was then less studied than other phases with 
which he has little to do. The fact probably is that no previous 
agent, in an equal period of time, has so greatly influenced the life 
of the land, or the rate of land-degradation, as man has since the 
agricultural epoch was well established. That this influence will 
be increased during coming centuries seems clear. The flora is 
rapidly passing from that which had been evolved by natural 
agencies through the ages, to that which man selects for cultivation 
or preservation. With the further progress of this movement, 
native floras seem destined to early extinction. The same may 
be said of native faunas. Favored animals, under man’s care, 
flourish beyond precedent, while others, so far as they are within his 
reach, are suffering rapid declines that look toward extinction. 


LIFE 633 


Life in the sea is less profoundly affected than that on the land, but 
even that does not escape modification. The most pronounced 
exceptions to man’s dominance, and those that bid fair to contest 
his supremacy longest, are found in organisms too minute to be 
controlled easily by him, and in organisms that, quite against his 
will, flourish on the conditions he furnishes. But even the acceler- 
ated evolution of these organisms is a part of the profound biological 
revolution which attends man’s dominance. 

Man’s control has not thus far been characterized by much 
recognition of the complicated interrelations of organisms and of 
the consequences of disturbing the balance in the organic kingdom, 
and he is reaping, and is certain to reap more abundantly, the 
unfortunate fruits of ignorant and careless action. For the most 
part, man has been guided by immediate considerations, and even 
these not always controlled by much intelligence. Thus great 
wantonness has attended his destruction of both plant and animal 
life. But a more intelligent as well as a more sympathetic attitude 
is developing, and will doubtless soon become dominant. A new 
era in control and selection is dawning. New varieties and races 
are being produced that not only depart widely from the parent 
stock, but diverge in lines chosen to meet given conditions, or to 
produce desired products. How far this may yet go it is impossible 
now to predict. 

Prognostic geology. The long perspective of the past should 
afford at least some suggestions of the future, but it must be con- 
fessed that the most important conjectures as to the future are 
dependent on interpretations of the past that are not yet certain. 
A word has been said relative to a possible return of a glacial epoch, 
but no sure prediction can be made. Question has been raised as 
to whether the deformations of recent times are over, but the 
answer remains uncertain. The duration of the earth as a habitable 
globe has been a common theme of prognosis. A final refrigeration 
as the result of the cooling of a once molten globe has been the usual 
forecast, and the final doom of the race has been a favorite theme for 
pseudo-scientific romances. But this all hangs on the doctrine of a 
former molten earth, if not on the doctrine of its origin from a gase- 
ous nebula. Under the alternative conception of a slow-grown earth 
conserving its energies, conjoined with a more generous conception 
of the energies resident in the sun and the stellar system, no narrow 
limit need be assigned to the habitability of the earth. A Psycho- 


684 THE HUMAN PERIOD 


zoic era, as long as the Cenozoic or the Paleozoic, or an eon as long 
as the cosmic and the biotic ones, may quite as well be predicted as 
anything less. ‘The forecast is at best speculative, but an optimistic 
outlook seerns more likely to prove true than a pessimistic one. An 
immeasurably higher evolution than that now reached, with attain- 
ments beyond present comprehension, is a reasonable hope. 

The forecast of an eon of intellectual and spiritual development 
comparable in magnitude to the prolonged physical and biotic 
evolutions lends to the total view of earth-history great moral 
satisfaction, and the thought that individual contributions to the 
higher welfare of the race may realize their fullest fruits by con- 
tinued influence through scarcely limited ages, gives value to life 
and inspiration to personal endeavor. 7 


APPENDIX 
REFERENCE TABLE OF THE PRINCIPAL GROUPS OF PLANTS. 


Alge and algoid 


forms 
THALLOPHYTES 
(Thallus plants) 
Fungi and fungoid 
forms 
Lichens 
BRYOPHYTES Hepatice, liverworts. 
(Moss plants) | Musci, mosses. 
Lycopodiales. 
Daernoeyrrs Sphenophyllales. 
(Fern plants) Equisetales 
Filicales 
Gymnosperme 
(naked seed) 
SPERMATOPHYTES 


(Seed plants) 
Angiospermze 


(covered seed) 
(Flowering plants) 


685 


Diatomacee, diatoms. 
Coccospheres \ p ee 
Rhabdospheres { ~ “*5!° *'8®- 
Cyanophycee, blue-green alge. 
Chlorophycee, green 
alge, including 
stoneworts. 
Rhodophycee, 
alge. 
Pheophycee, brown 
alge. 
Myxomyeetes, 
slime-molds. 
Schizomycetes, 
bacteria. 
Phycomycetes, 
molds. 
Ascomycetes, 
dews. 
Basidiomycetes, basidium-fungi, 
smuts, rusts, mushrooms. 
Symbiont alge and fungi. 


red >; True alge. 


‘animal fungi,” 


*fission-fungi,” 
algze-fungi, water- 


ascus-fungi, mil- 


Lepidodendra, _ sigillarias, club- 


mosses. 


( Calamites. 
{ Equisete, 
| — tails. 

Filices, true ferns. 

Cycadofilicales. 

Bennettitales. 

Cycadales. 

Cordaitales. 

Ginkgoales. 

Coniferales. 

Gnetales. 

Dicotyledones. Most common 
forest trees (except conifers), 
most shrubs and most netted- 
veined leaved herbs. 

Monocotyledonez, cereals, grasses, 
eLc. 


scouring-rushes, horse- 


686 


APPENDIX 


REFERENCE TABLE OF THE PRINCIPAL GROUPS OF ANIMALS! 


PROTOZOA (the 
simplest animals) 


COELENTERATA Porifera 
(Sponges, corals, 
jellyfishes) Cnidaria 
Pelmatozoa 
ECHINODERMATA 
(Crinoids, star- Asterozoa 
fishes, sea-urchins) 
Echinozoa 
VeERMES (Worms) 
MOLLUSCOIDEA 
(Mollusc-like forms) 
MOoOLuLusca 
(Molluscs) 
Branchiata 
ARTHROPODA 
(The articulates) 
Tracheata 





VERTEBRATA 


1 After Zittel in the main. 


| 
| 
| 
| 


{ 


Rhizopoda Foraminifera. 
Radiolaria. 
rarliniay Unknown in 
Gregarina fossil state. 
Spongize { Calcareous sponges. 


\ Siliceous sponges. 

Anthozoa, coral polyps. 

Hydrozoa, hydroids and medusz. 

Cystoidea, cystids. | 

Crinoidea, stone lilies. 

Blastoidea, blastoids, 

Ophiuroidea, brittle-stars. 

Asteroidea, starfishes. 

Echinoidea, sea-urchins. 

Holothuroidea, sea-cucumbers. 

Platyhelminthes 

Rotifera 

Nemathelminthes 

Gephyrea 

Annelida, sea-worms. 

Bryozoa, sea-mosses. 

Brachiopoda, lamp-shells. 

Pelecypoda, lamellibranches, bivaives. 

Scaphopoda, tusk-shells. 

Amphineura, chiton. 

Gastropoda, univalves, snails, etc. 

Cephalopoda, nautilus, cuttlefish. 

Crustacea. 

Trilobita, trilobites. 

Gigantostraca, horseshoe crabs. 

Entomostraca, ostracoids, barnacles. 

Malacostraca, lobsters, crabs. 

Myriapoda, centipedes. 

Arachnoidea, spiders, scorpions. 

Insecta, insects. 

Cyclostomata, lampreys. 

Selachii, sharks. 

Holocephali, spook-fishes. 

Dipnoi, lung-fishes. 

Teleostomi, ganoids and 
| teleosts(common fishes). 

Amphibia, amphibians, batrachians. 

Reptilia, reptiles. . 

Aves, birds. 


Rare as fossils. 


Pisces 
(fishes) 


Wintnnehn Prototheria, monotremes. 
(mammals) Metatheria, marsupials. 
Eutheria, placentals. 


INDEX 


Abrasion by wind, 22 
Acadian epoch, 344, 346 
Accidents to streams, 101-107 
Acidic rocks, 252 
Acondylacanthus gracilis, 433 
Acrocrinus amphora, 436 
Acrotreta gemma, 360 
Actzon shilohensis, 597 
Actinocrinus lobatus, 434 
Actinocrinus senectus, 433 
Actinolite, 255 
Actinopteria textilis, 414 
Adams, F. D., cited, 240 
Adirondacks, Proterozoic rocks of, 340 
Adjustment of streams, 97, 98 
Africa, Cretaceous in, 544 
Devonian in, 412 
Miocene of, 590 
Pennsylvanian in, 457 
Permian in, 474 
Triassic in, 493 
Aftonian epoch, 633 
Agassiz, A., cited, 167 
Agassizocrinus dactyliformis, 436 
Agates, 254, 286 
Age of earth, 168 
Agglomerate, 250, 258, 263 
Aggradation by streams, 107 
Agnostus interstrictus, 359 
Alabaster, 257 
Algonkian. See Proterozoic. 
Allegheny series, 443 
Alluvial cones, 108, 109 
deposits, 108, 660 
fans, 109, LIO 
plains, 111 
terraces, 119 
Alpine glaciers, 127 
Amazonstone, 254 
Amber, 577 
~ Amberleya dilleri, 531 
Amethyst, 254 
Ammonites, 499 
Jurassic, 509 
Amphibians, Eocene, 572 
Pennsylvanian, 464 


Amphibians — continuea 
Permian, 477 
rise of, 464 
Triassic, 494 
Amphibole, 253, 255 
Amygdaloid, 259 
Anatina austinensis, 530 
Anchippus, 579, 592 
Anchisaurus colurus, 495 
Anderson, R., cited, 583 
Angiosperms, Comanchean, 528 
place of origin, 528 
Animals, classification of, 586 
Animikean, 331 
Annelids. See worms. 
Annularia sphenophylloides, 461 
Anomalina ammonoides, 572 
Anomalocrinus incurvus, 384 
Antecedent drainage, 105 
Anthracite coal, 453 
Anthracite coal field, 443 
Anthrapalemon gracilis, 466 
Anticlinal fold, 274 
Anticlinoria, 220, 277 
Antiquity of man, 670-676 
Apennines, Pliocene in, 608 
Aperiodic movements, 219 
Aphanite, 248, 259, 262 
Aphorrhais prolabiata, 554 
Appalachian coal field, 443 
Appalachian drainage, 105 
Appalachian Mountains, age of, 475 
Aqueous metamorphism, 286 
Arabellites cornutus, 386 
ovalis, 386 
Arapahoe formation, 539 
Arca staminea, 595 
tehamaensis, 531 
Archeopteris bochsiana, 460 
Archeopteryx macrura, 519 
Archean, bearing on origin of earth, 323 
delimitations, 317 
distribution of, 320, 322 
general characteristics, 317 
granites, 318 
origin of, 318, 320 


687 


688 INDEX 


Archean — continued Aucella crassicollis, 531 
rocks, 316, 317 mosquensis, 510 
schists, 318 piochii var. orata, 531 
Archecyathus minganensis, 386 Augite, 255 
rensselericus, 362 Augitite, 260 
Archeocrinus desideratus, 384 Augusta series, 426, 427 
Archeozoic eon, 308 Aurochs, 675 
Archeozoic era, 314 Australia, Cambrian glacial beds in, 
climate of, 324 357 
duration of, 324 Carboniferous in, 457 
life of, 324 coal in, 457 
Archimedes, 438 Cretaceous in, 527, 545 
swallovanus, 437 glacial Permian in, 474 
Archinacella cingulata, 381 Miocene of, 590 
Argillite, 292 Autoclastic rocks, 295 
Arid regions, erosion in, 83 Aviculopecten occidentalis, 469, 481 
Aridity, Permian, 470 Azoic eon, 307 
Silurian, 392, 393 
Arietide, 508 Baculites, Cretaceous, 555 
Arikaree formation, 588 grandis, 554 
Aristozoe rotundata, 359 Bad lands, 83, 85, 576 
Arkose, 266 Bain, H. F., cited, 420 
Arnold, R., cited, 558, 574, 583, 604, | Ball, Sir Robt., cited, 649 
611 Barker, A. S., cited, 167 
Artefac, 670 Barnacles, 418 
Artesian wells, 52 Barrier, 184 
Arthracantha punctobrachiata, 419 Bars, 186, 187 
Arthrodirans, 418, 422 Barus, Carl, cited, 33 
Arthrolycosa antiqua, 466 Barycrinus hoveyi, 434 
Artiodactyls, 568 Basal conglomerate, 328 
Ashley, G. H., cited, 607, 661 Basalt, 260, 262 
Asia, Cambrian in, 356 Base-level, 66, 68, 69 
Carboniferous in, 457 Base-leveled plain, 68 
Cretaceous in, 527, 544 Basement complex, 319 
Devonian in, 412 Basic rocks, 252 
Jurassic of, 507 Bastin, E. S., cited, 642, 666 
Miocene of, 590 Batholiths, 228 
Ordovician in, 376 Batocrinus, 434 
Pennsylvanian in, 457 Bays sandstone, 369 
Permian in, 474 Beach, 184 
Triassic in, 492 Beadnell, H. J. L., cited, 19 
Astarte californica, 531 Becraft limestone, 402 
thomasii, 595 Beekmantown limestone, 368 
Asteroids, 2 Belemnites, 509 
Astral eon, 307 densus, 512 
Astresius liratus, 531 Bellerophon clausus, 381 
Athyris lamellosa, 435 percarinatus, 469 
Atlantic coast, submergence of, 171 sublevis, 437 
Atmosphere, 4 Berry, E. W., cited, 611 
beginning of, 310 Betulites westi, var. subintegrifolius, 
composition of, 4 547 
thermal effects of, 28 Bifidaria armifera, 644 
under nebular hypothesis, 309 corticaria, 644 
work of, 12-29 muscorum, 644 


Atrypina imbricata, 414 pentodon, 644 


Billingsella coloradoensis, 360 
transversa, 360 
Bilobites varicus, 414 
Biotite, 255 
Birds, Cretaceous, 550, 5 51 
Eocene, 572 
Jurassic, 510 
Bison, 666 
Black Hills, 340 
Black River limestone, 368 
Blastoids, Pennsylvanian, 468 
Blue mud, 198 
Bonney, T. G., cited, 245 
Botriopygus alabamensis, 554 
Bowlder-clay, 618 
Bowlders in drift, 617 
Brachiopods, Cambrian, 360 
Devonian, 418 
Jurassic, 512 
Mississippian, 414, 415, 419 
Ordovician, 378, 382 
Pennsylvanian, 468 
Silurian, 396, 397 
Triassic, 499 
Brachiosaurus, 517 
Brachiospongia digitata, 386 
Branner, J. C., cited, 373, 564 
Brazil, coal in, 457 
Breccia, 267 
Brontosaurus (Apatosaurus), 517 
Bronzite, 255 
Brooks, A. H., cited, 447 
Brooksella alternata, 362 
Broom, R., cited, 479 
Brule formation, 575 
Bryozoans, Devonian, 414, 419 
Mississippian, 435 
Ordovician, 383 
Pennsylvanian, 468 
Silurian, 398 
Buchanan epoch, 633 
Buckley, E. R., cited, 34, 122, 374 
Bumastus trentonensis, 378 
Bunter formation, 491 
Buttes, 93, 94 
Bysmaliths, 228 


Cairngorm, 254 

Calamites, 424, 459, 476 
Calamites cistii, 461 
Calcareous tufa, 660 

Calcite, 256 

Calhoun, F. H. H., cited, 625 


California earthquake, 208, 209, 217 


rift, 583 


INDEX 689 


Calliostoma philanthropus, 597 
Callipteridium membranaceum, 466 
Callopora pulchella, 383 
Caloosahatchie beds, 604 
Calvert, W. R., cited, 539 
Calvin, S., cited, 645, 665 
Calymene callicephala, 378 
niagarensis, 399 
Camarotoechia barrandei, 415 
Cambrian faunas, origin of, 366 
succession of, 364 
Cambrian glaciation, 356, 357 
Cambrian life, 358 
advancement of, 363 
Cambrian of Europe, 356 
Cambrian period, 344-366 
close of, 355 
duration of, 357 
Cambrian rocks, distribution, 345, 347 
metamorphism of, 354, 355 
outcrops, 352 
Cambrian sedimentation, 351 
submergence, 349 
Campbell, M. R., cited, 97 
Campeloma harlowtonensis, 530 
Camptonectes bellistriatus, 512 
Canada, Archean rocks of, 321 
Proterozoic rocks of, 340 
Canadian epoch, 368 
Cancellaria alternata, 597 
Canoe-shaped valleys, 95 
Canyon of the Yellowstone, 70 
Canyons, 70, 87 
Carabocrinus vancortlandti, 384 
Carbon formation, 539 
Carbonaceous slates in Huronian, 332, 
ak glee 
Carbonation, 23 
of igneous rocks, 264 
Carboniferous. See Pennsylvanian. 
Cardioceras cordiformis, 512 


‘Cardium leptopleurum, 595 


Carnivores, Eocene, 570 
Miocene, 594 
Caryocrinus ornatus, 395 
Cascade formation, 525 
Cascade Mountains, age of, 588 
Cassidulus subquadratus, 554 
Catazyga headi, 382 
Cat’s-eye, 254 
Catskill formation, 402, 406 
Caverns, 38, 390 
Cavity filling, 286 
Cayugan epoch, 388 
series, 391 


690 


Cepnalaspis, 421 
Cephalopods; Cambrian, 361 
Cretaceous, 554 
Devonian, 417 
Jurassic, 512 
Mississippian, 433, 437 
Ordovician, 378-380 
Pennsylvanian, 468 
Permian, 480, 481 
Silurian, 397 
Triassic, 498, 490 
Ceratites nodosus, 498 
whitneyi, 500 
Ceratops beds, 539 
Ceratosaurus nasicornis, 516 
Ceraurus pleurexanthemus, 378 
Cerithium paskentensis, 531 
Cerithium (?) texanum, 530 
Cetaceans, 571 
Chadron formation, 575 
Chalk, 267 
origin of, 537 
Chamberlin, R. T., cited, 226, 241, 310, 


451 
Chamberlin, T. C., ‘cited, 43, 52, 131, 
226, 337, 374, 376, 628, 630, 632, 645 
Champlain epoch, 633 
Champsosaurus, 551 
Changes of level, effect on streams, ror 
during Pleistocene, 661, 662 
Charleston earthquake, 210 
Chautauquan series, 402 
Chazy limestone, 368 
Chemical deposits in sea, 193, 199 
sediments, 268 
Chemung fauna, 420 
formation, 402, 406 
Chert, 268, 288 
Chesapeake Bay, 106 
Chesapeake formation, 581 
Chester series, 426, 427 
Chickamauga limestone, 360 
Chico series, 526, 540 
Chief Mountain, 542 
Chimney-rocks, 179 
China, Cambrian glacial beds in. 357 
coal in, 457 
Chloritic rock, 291 
Chlorite, 256 
Chonetes cornutus, 396 
coronatus, 419 
granulifera, 460 
Choristoceras marshi, 4098 
Chrysolite, 255 
Cidaris coronata, 511 


INDEX 


Cincinnati Arch, 373° 
Cincinnatian epoch, 36% 
Cinder-cones, 236 
Cladoselache fyleri, 439 
Cladodus springeri, 433 _ 
Claosaurus annectens, 544: 
Clark, W. B., cited, 532 
Classification of rocks, 297 
Clastic rocks, 267, 269 
Clear Fork limestone, 471 ‘ 
Cleland, H. F., cited, ror a) 
Cliff glaciers, 130 = 
Cliffs in arid regions, 83, 94 
Climacograptus bicornis, 385 
Climate, Cambrian, 356 

Comanchean, 528 

Cretaceous, 545 

Devonian, 413 

effect on erosion, 81 

Eocene, 564 

Jurassic, 507 

Miocene, 591, 598 

Ordovician, 376 

Pennsylvanian period, 463 

Permian, 476 

Pleistocene, 613, 648 

Pliocene, 611 

Proterozoic, 342 

Quaternary, 613, 648 

Silurian, 393 

Triassic, 490 
Climates of past, criteria of, 195 
Clinton beds, 388, 380 

iron ore, 389 
Coal, 269, 447,448, 486, 507, 538, 5590, 560 

Eocene, 559 

Jurassic, 506, 507 

Laramie, 538 

Pennsylvanian, 443 et seq. 

varieties of, 453 
Coal-beds, extent of, 453 
Coal fields, 443-445 
Coal measures, 441 
Coastal plain, structure of, 524 
Coast lines. See shore lines. 
Cobalt in Huronian rocks, 334, 337 
Cobleskill limestone, 388 
Coccosteus decipiens, 423: 
Coeymans limestone, 402 
Coleman, A. P., cited, 342, 665 
Coleopters, 467 
Colorado Canyon, 88 

age of, 606 
Colorado River, delta of, 116, 118 © 
Colorado series, 535, 536 


INDEX 


Columbia River, 107 

Columbia series, 652 

Columnar structure, 247, 248 
Columnaria alveolata, 385 
Comanchean angiosperms, 528 
Comanchean, distinct from Cretaceous, 


521 
Comanchean fossils, 528 
Texan, 530 
Comanchean of Mexico, 525 
Comanchean period, 521-531 
close of, 526 
Comanchean system, distribution of, 
522 
Comarocystis punctatus, 384 
Compound alluvial fan, 110 
Concretions, 287, 288 
in Cretaceous, 536 
Condylarthra, 566, 567 
Conemaugh series, 443 
Conglomerate, 267, 271 
great thicknesses of, 191 
Conifers, Jurassic, 515 
Conocardium prattenanum, 437 
trigonale, 416 
Consequent streams, 61 
Constellaria polystomella, 383 
Continental creep, 350 
glaciers, 127 
shelves, 171 
Continent-forming muvements, 221 
Contour maps, 17 
Conularia trentonensis, 381 
Conus diluvianus, 597 
Cook, G. H., cited; 523 
Copper in Michigan, 337 
Coral reefs, oldest, 391 
Corals, Cambrian, 362 
Cretaceous, 555 
Devonian, 414, 417 
Jurassic, 509 
Mississippian, 415, 420 
Ordovician, 384, 385 
Pennsylvanian, 468 
Silurian, 398 
Triassic, 500 
Corbula aldrichi, 573 
blakei, 500 
idonea, 595 
Corbula (?) persulcata, 531 
Cordilleran ice sheet, 613 
Cordilleran Mountains, age of, 541 
region, Proterozoic rocks of, 340 
Cordaites, 424, 458, 459, 462, 476 
Coroniceras bisulcatum, 508 


691 


Corniferous formation, 402 
Corrasion, 73, 78 
Correlation, basis of, 346-349 
Corrosion, 73, 80 
Cotylosauria, 478 
Cowles, H. C., cited, 20 
Crag and tail, 156 
Crania loelia, 382 
Crassatellites alaeformis, 574 
marylandicus, 595 
Craters, 230 
Crazy Mountains, 543 
Creep, 40, 75 
continental, 350 
Creodonta, 566, 567, 570 
Crepidula fornicata, 597 
Crepipora hemispherica, 383 
Cretaceous fossils, 554 
Cretaceous life, 546 
Cretaceous period, 521, 532-555 
climate of, 545 
close of, 541, 542 
Cretaceous plants, 546, 547 
saurlans, 546 
Cretaceous system, 533 
structure of, 532, 535, 540 
Crevasses in glaciers, 135 
Crinoids, Cretaceous, 555 
Devonian, 418 
Jurassic, 509 
Mississippian, 437 
Ordovician, 383 
Silurian, 396 
Cristellaria gibba, 572 
radiata, 572 
Criteria of glaciation, 616 
Critical level, 170, 455 
Crocodiles, Cretaceous, 540 
Crocodilians, 514, 519 
Croll, James, cited, 649 
Cross, Whitman, cited, 40, 489, 504, 
538, 539 
Cross-bedding, 192, 270, 271 
Crustacea, Cambrian, 359 
Jurassic, 510 
Crustal movements, amount of, 224 
cause of, 224 
Crustal shortening, 223 
Cryphzus boothi, 419 
Crystals, growth in lava, 251 
Crytina hamiltonensis, 416 
Ctenodonta nasuta, 381 
pectunculoides, 381 
recurva, 381 


| Culm, 431 


692 


Currents in streams, 77 
Currents, ocean, 188 
Cushetunk Mountain, 94 
Cut-offs, 114 
Cycadeoidea dakotensis, 529 
Cycads, Comanchean, 529 
Jurassic, 515 
Cycle of erosion, 66, 103 
stages in, 69 
Cyclomena bilix, 381 
Cyphaspis christyi, 399 
Cypricardella bellistriatus, 419 
Cyrtoceras neleus, 380 
Cyrtolites ornatus, 381 
Cystids, 362 
Ordovician, 383 
Pennsylvanian, 468 
Silurian, 396 


Dedicurus clavicaudatus, 669 
Dakota formation, 535, 536 
Dall, W. H., cited, 556,574, 581, 506, 611 
Dalmanella testudinaria, 382 
Daly, R. A., cited, 103 
Dana, J. D., cited, 179, 226, 245, 307 
Darton, N. H., cited, 575 
David, T. W. E., cited, 357, 413 
Davis, B. M., cited, 37 
Davis, C. A., cited, 202 
Davis, W. M., cited, 13, 68, 118, 487, 
560, 606, 628, 645 
Dawson, G. M., cited, 429, 526, 641 
Dawson, J. W., cited, 641 
Dead Sea, 204 
Debris in ice, +53 
Deccan lava flows, 544 
Deep-sea deposits, 189, 196 
Deer, earliest, 592 
Degradation, rate of, 84 
Dehydration, 291, 292 
Deiphon forbesi, 399 
Delaware River, 105 
Delaware Water-Gap, 92 
Delta fingers, 117 
Delta of the Mississippi, 115 
Deltas, 108, 116, 117 
Dendrocrinus polydactylus, 384 
Dentalium attenuatum, 597 
Denver formation, 539 
Deposition by shore-currents, 184 
by streams, 107 
by undertow, 184 
by waves, 184 
of drift, 156 
of mineral matter from solution, 37 


INDEX 


Deposits in sea, 172, 189 
Deposits by springs, 652 
Derbyia crassa, 469 
Deroceras subarmatum, 508 
Desert sandstone, 545 
Des Moines series, 443 
Devonian, climate of, 413 
Devonian fauna in Great Basin, 420 
fishes, 416, 418 
floras, 423 
igneous rocks, 410 
Devonian land life, 421, 423 
life, 413 ? 
Devonian oil and gas, 410 
Devonian period, 402-425 
close of, 410 
Devonian phosphates, 411 
system, outcrops of, 410 
Diabase, 261 
Diallage, 260 
Diastrophic movements, periodic, 219, 
220 
Diastrophism, 2, 170, 206 
of Archeozoic era, 318 
of Jurassic, 505 
of Middle Miocene, 584 
of Permian, 475 
of Quaternary, 661 
Diatom 00ze, 199 
Dichocrinus inornatus, 433 
Dichograptus octobrachiatus, 385 
Dicranurus hamatus, 414 
Dictyopteris rubelia, 460 
Didymograptus nitidus, 385 
Dielasma bovidens, 469 
Dikellocephalus fauna, 349 
Dikellocephalus pepinensis, 348 
Dikes, 228 
Diller, J. S., cited, 586, 606, 663 
Dinichthys herzeri, 418 
Dinoceras mirabile, 569 
Dinocerata, 568 
Dinosaurs, 494 
Cretaceous, 546 
Jurassic, 515 
Dinotherium, 610 
Diorite, 260, 261, 262 
Dip, 275 hee 
Diplograptus pristis, 385 
Diplopodia texanum, 530 
Dipnoi, 422 
Dipterus valenciennesi, 423 
Discorbina turbo, 572 
Divides, permanent, 63 
Dolerites, 262 


INDEX 


Dolomite, 196, 257, 267, 491 
Dolomization, 267 
Don River beds, 665 
Dosiniopis lenticularis, 573 
Double Mountain formation, 471 
Drainage, affected by glaciation, 632 
Drake, N. F., cited, 561 
Drepanochilus nebrascensis, 554 
Drift, 8, 151 

constitution of, 616 

contact with rock, 620 

deposition of, 160 

distribution of, 618 

structure of, 618 

thickness of, 620 

topography of, 620 

transportation of, 153 
Drowning of valleys, 106 
Drumlins, 627, 629 
Dumble, E. T., cited, 600 
Dunes, 15, 655 

distribution of, 21 

migration of, 19 

topography of, 16 
Dust, eolian, 12 

volcanic, 13 
Dutton, C.E., cited, 83, 206, 226, 245,604 
Dwyka conglomerate, 474 
Dynamic metamorphism, 291 


Earth, constitition of, 315 
form of, 6 
mass of, 3 
origin of, 299 
size of, 6 
specific gravity, 11 
Earth history, stages of, 313 
Earthquake fissures, 212 
vibrations, 208 
waves, 210 
Earthquakes and changes of level, 216 
and faults, 214 
Earthquakes, distribution of, 211 
effect on life, 215, 216 
geologic effects, 207, 211 
Eastern Interior coal field, 444 
Eatonia medialis, 414 
Eccyliomphalus triangulus, 381 
Echinocaris punctata, 419 
Echinoderms, Cretaceous, 554, 555 
Jurassic, 509-511 
Mississippian, 436 
Ordovician, 383, 384 
Silurian, 395 
Triassic, 499 


eeaees 2 


Echinoids, Silurian, 306 
Ecphora quadricostata, 597 
Ectonocrinus grandis, 384 
Edentates, Eocene, 571 
Eldridge, G. H., cited, 583 
Electricity, effects of, 29 
Elephants, Miocene, 592 
Elephas antiquus, 669 
Eleutherocrinus cassedayi, 419 
Elevated barrier beach, 185 
Ellensburg formation, 607 
Elotheres, 579 
Empire beds, 585 
Enchanted Mesa, 93 
Endothyra baileyi, 437 
Englacial drift, 147, 153 
Ensis directus, 595 
Enteletes hemiplicata, 469 
Entrenched meanders, 102 
Eocene carnivores, 570 
Eocene, close of, 564 
geography of, 564 
Eocene coal, 559 
Eocene flora, 566 
Eocene in South America, 564 
in West Indies, 564 
Eocene life, 565 
mammals, 565, 566 
mollusks, 573 
Eocene of western interior, 559 
Eocene period, 556-580 
Eocene system, 556 
composition of, 558 
in the west, 558 
thickness of, 558 
Eolian deposits, 652 
sand, I5 
Eoscorpius carbonarius, 466 
Eotrocus concavus, 437 
Epeirogenic movements, 218 
Epicontinental seas, 5 
Equisetales, 458 
Eretmocrinus remibrachiatus, 434 
Erian series, 402 
Erosion, affected by climate, 81 
analysis of, 73-80 
conditions affecting, 80 
cycle of, 66 
Erosion by glaciers, 147 
by running water, 59 
by waves, 176 
Erosion in arid regions, 83 
in Mississippi basin, 84 
Erosion of folds, 95 
Eskers, 164, 165 


693 


604 


Esopus grit, 402 
Eucalyptocrinus crassus, 395 
Euconulus fulvus, 644 
Kugnathus athostamus, 513 
[Eumetria marcyi, 437 
Eunicites gracilis, 386 
varians, 386 
Eupachycrinus magister, 469 
Euphoberia armigera, 466 
Europe, Cambrian of, 356 
Carboniferous of, 430, 456 
Cretaceous of, 527, 542 
Devonian of, 411 
Eocene of, 562 
Jurassic of, 506 
Miocene of, 588 
Oligocene of, 576 
Ordovician of, 375 
Permian of, 472 
Pliocene of, 608 
Silurian of, 394 
Trias of, 401 
European ice sheet, 615 
Eurypterus, 400 
Eurypterus fischeri, 401 
mansfieldi, 467 
Eutaw formation, 534 
Evanston formation, 539 
Evaporation, 28 
Exfoliation, 265 
Exogyra (Ostrea) virgula, 510 
Extinct lakes, 202 


Extra-terrestrial deposits in sea, 198 


Falls, 89 
Fanfold, 276 
Faulting in western mountains, 541 
Faulting, Quaternary, 659 
Faults, 281 
and earthquakes, 214 
and folds, 282 
distributive, 283 
hade of, 282 
heave of, 282 
normal, 281 
significance of, 283 
throw of, 282 
thrust, 282 
Fault-scarp, 282 
Fauna defined, 346 
Favosites occidens, 3098 
Feldspar, 253, 254 
Felsites, 262 
Fenestella emaciata, 419 
parvulipora, 398 


INDEX 


Fenneman, N. M., cited, £73 
Ferns, Devonian, 423 ~ 
Pennsylvanian, 450, 450 
Ficus inequalis, 547 
Fiords, 152, 153 
Fisher, O., cited, 226 
Fishes, Cretaceous, 554 
Devonian, 418 
Jurassic, 510, 512, 513 
Mississippian, 438-449 
Onondagan, 415 
Ordovician, 386 
Silurian, 399, 401 
Fissure eruptions, 229 
Fissuridea alticosta, 597 
griscomi, 597 
Flaxseed iron ore, 289 
Flints, 268 
Flood plain deposits, .111 
Flood plains, material of, 113 
Floods, 57 
Florissant beds, 575 
Flow structure, 247, 248 
Flowing wells, 54 
Fluviatile deposits, 652 — 
Fluvio-glacial deposits, 631 
Folding, 220 
Folds, erosion of, 95 
Foliation, 294 
Foliation of ice, 126 
Foraminifers, Cretaceous, 555 
Eocene, 572 
Forbesiocrinus wortheni, 434 
Fordilla troyensis, 361 
Formation, defined, 269 
Fort Union series, 539 
Fossils, 267 
Fraas, E., cited, 575 
Fredericksburg formation, 524 
French Broad River, 105 
Fulgar spiniger, 507 
Fuller, M. L., cited, 34 
Fusulina limestone, 457 
Fusulina secalicus, 468, 469 
Fusus (?) interstriatus, 573 
texanus, 530 


Gabbroids, 263 

Gabbros, 260, 262 

Galena limestone, 369 
Gangamopteris cyclopteroides, 478 
Gangamopteris flora, 477 

Ganoids, 422 

Garnet, 257 

Gas and oil in Ohio, 374 


INDEX 


Gas and oil, Ordovician, 374 
Gases in igneous rocks, 241 
Gases of volcanoes, 236 
Gastropods, Cambrian, 361 
Cretaceous, 554, 555 
Devonian, 417 
Miocene, 597 
Ordovician, 380, 381 
Silurian, 397 
Triassic, 499 
Geanticlines, 277 
Geikie, Sir A., cited, 226, 245, 356 
Geikie, J., cited, 160, 171, 668, 676 
Genesee shale, 402 
Geodes, 286 
Geology defined, 1 
Geology, subdivisions of, 1 
time divisions, 323 
Georgian epoch, 344 
Geosaurus suevicus, 515 
Geosynclines, 277 
Geysers, 49 
(ilbette <a. K., cited, 73, 91, 118, 173, 
203, 226, 646, 661, 676 
Glaciated areas, re-peopling, 678 
Glaciated regions, soil in, 680 
Glaciated rock surfaces, 161 
Glaciation and drainage, 632 
Glaciation, criteria of, 616 
early, 309 
effect on life, 678 
Glacial climate, cause of, 648 
Glacial debris, 136, 137 
nature of, 150 
Glacial deposits, nature of, 160 
Glacial drainage, 138 
Glacial epochs, 632, 633 
Glacial erosion, topographic effects of, 


151 
Glacial formations, Cambrian, 356, 357 
Permian, 473 
Proterozoic, 342 
Glacial period, 613 
duration of, 646 
effect on life, 663 
end of, 677 
time since, 646, 647 
Glacial planation, 622 
Glacial soils, 680, 681 
Glacial striz, 148, 161, 622 
arrangement of, 623 
Glacial tables, 137 
Glacter ice, structure of, 139 
Glacier motion and _ transportation, 


144 


695 


Glaciers, development of, 140 
erosion by, 147 
fluctuations of, 133 
limits of, 132 
movement of, 126, 132, 133, 140, 142, 
146 
Pleistocene,in western mountains, 657 
structure of, 125, 139, 155 
texture of, 126 
transportation by, 140 
types of, 127 
upturning of layers, 155 
Glaciers and rivers contrasted, 134 
Glacio-fluvial deposits, 139, 164 
Glacio-lacustrine stage, 633, 636 
Glauconite, 532 
origin of, 532 
Globigerina bulloides, 572 
Globigerina ooze, 199 
Glossopteris angustifolia, 478 
communis, 478 
Glossopteris flora, 475, 477, 478 
Glycerites sulcatus, 386 
Glycimeris idoneus, 573 
Glyptocrinus decadactylus, 384 
Glyptodon, 669 
Golden Gate series, 505 
Goldthwaite, J. W., cited, 606 
Goniatites, 418 
Goniatites kentuckiensis, 437 
vanuxemi, 419 
Goniobosis (?) ortmanni, 530 
Goniophyllum pyramidale, 398 
Gorges, 88 
Graben, 221 
Gradation, 2 
cause of submergence, 340 
in sea, 172 
Grade, 66, 68 
Grammatodon inornatus, 512 
Grammysia hannibalensis, 433 
Grand Canyon, 86 
Precambrian rocks of, 341 
Grand Canyon group, 341 
Grand Gulf formation, 574 
Granites, 259, 262 
Granitic rock, 249 
Granitoids, 263 
Grant, U. S., cited, 374 
Graphic granite, 259, 260 
Graphite, 257 
Graptolites, 377 
Cambrian, 362 
Ordovician, 385 
Silurian, 399 


696 


Grasses, beginning of, 546 
Great Basin fauna, 436 
Great Salt Lake, 204, 657 
Great sea-wave, 209 
Greenland ice sheet, 125, 615 
Green mud, 198 
Greensand marl, 532 
Greenstone, 261 
Ground moraine, 159, 627 
Ground water, 30-55 

amount of, 33 

depth of, 32 

fact of, 30 

fate of, 34 

movement of, 33 

surface, 31 

work of, 35, 38, 45 
Gryphea arcuata, 510 
Guelph dolomite, 388 
Gulf of Suez, 608 
Gullies, 58 

growth of, 60 
Gulliver, F. P., cited, 182 
Gymnosperms, 458 
Gypidula galeata, 414 
Gypsum, 195, 257, 268 


Hadrosaurus mirabilis, 548 
Halysites catenulatus, 398 
Hamilton fauna, 418, 419 


Hamilton formation, 402, 404, 407 


Hanging valleys, 151, 152 
Harris, G. D., cited, 581 


Hartnagle, C. A., cited, 368, 389 


Hatcher, J. B., cited, 578, 590 
Hawaii, volcanoes of, 234 
Haworth, E., cited, 586 
Hay, O. P., cited, 679 
Hayes, C. W., cited, 374, 583 
Head erosion, 60 

Hevertella sinuata, 382 
Hematite, 255 

Helderberg fauna, 413, 414 
Helderbergian series, 402, 403 
Helicoceras stephensoni, 554 
Helicotoma planulata, 381 
Heilprin, A., cited, 245 
Hemiaster dalli, 530 
Hemipters, 467 

Herbivores, Pliocene, 610 
Hercynian fauna, 413 
Hershey, O. H., cited, 604 
Hesperornis, 551, 552 
High-latitude glaciers, 127 
Hilgard, E. W., cited, 600 


INDEX 


Hill, Arthur, cited, 112 
Hill, R. T. cited, 564, 574 
Hipparion, 592 
Hipparionyx proximus, 415 
Hippurite limestone, 544 
Howe, E., cited, 480 
Hobbs, W. H., cited, 206 
Hogbacks, 93, 536 
Holaster simplex, 530 
Holmes, W. H., cited, 671 
Holmia bréggeri, 359 
Holocystites adiapatus, 395 
Holograptus richardsoni, 385 
Homo diluvii testis, 596 
Homomya austinensis, 530 
Hoplites angulatus, 531 
Hormotoma gracilis, 381 
Hornblendite, 260, 262 
Horse, evolution of, 592, 593, 610 
Horsetown series, 525 
Howchin, W., cited, 357 
Hudson River shale, 369 
Hudson Valley, 171 
Huntington, E., cited, 606 
Human period, 677 
Huronian iron ore, 332 
Huronian systems: ‘of rocks, 331, 334 
Hustedia mormoni, 469 
Hybocrinus tumidus, 384 
Hydration, 23, 264 
Hydrosphere, 4 

beginning of, 311 
Hyolithes americanus, 361 
Hypsipleura gregaria, 531 
Hyracotherium venticolum, 569 


Ice in soil, 121 
Ice on lakes, 121 
on rivers, 122 
on sea, 124 
Icebergs, 166 
Ice-cap of Greenland, 125 
Ice ramparts, 122 
Ice sheets, development of, 624 
slope of, 625 
succession of, 632 
thickness of, 624 
work of, 626 
Ice sheets of North An 613, 614 
Ichthyornis, 551 
Ichthyornis victor, 552 
Ichthyosaurs, 501 
Cretaceous, 550 
Jurassic, 513 
Ichthyosaurus quadriscissus, 513 


Idonearca nebrascensis, 554 
Igneous eruptions, Pleistocene, 657 


Igneous rocks, 9, 246 
classification of, 258 
composition, 251 
Cretaceous, 542 
Devonian, 410 
disruption of, 263 
Eocene, 561 
Huronian, 332 
Keweenawan, 334 
Miocene, 587 
Mississippian, 429 
occurrence, 246 
Ordovician, 370 
Silurian, 393 
structure of, 246 

Tlecillewzet Glacier, 158 

Illinoian drift, 634 

Illinoian ice sheet, 633 

Timenite, 260 

Ilyanassa (?) procina, 597 

Immediate run-off, 58 

Imperial Valley, 118 


India, glacial Permian in, 474 
Inoceramus vanuxemi, 554 


Insectivores, Eocene, 571 


Insects, Devonian, 424, 425 


Eocene, 572 
Oligocene, 578 
Ordovician, 387 
Pennsylvanian, 467 


Interglacial beds, Toronto, 664, 665 
Interglacial epochs, 633, 664 


Interior heat, 225 
Intermediate rocks, 252 
Intermittent streams, 58 
Intrusive rocks, 227 
Ione formation, 586 
Iowan drift, 635, 664 
ice sheet, 633 
Iron ore, 269 
Archean, 333 
Clinton, 43, 389 
Huronian, 332 
Lake Superior, 43, 332 
Pennsylvanian, 454 
Tron oxides, 254 
Ischadites, 386 
Isocardia markoéi, 595 
Isoclinal fold, 276 
Isocline, 275 
Isotelus maximus, 378 


James River, 105 


INDEX 697 


Jasper, 254 
Jelly fish, Cambrian, 362 
Jerseyan drift, 633 
John Day Basin, 575 
Johnson, L. C., cited, 600 
Joints, 278-281 
influence on erosion, 99 
Judd, J. W., cited, 245 
Jurassic ammonites, 508 
coal, 507 
fossils, 507 
plants, 515 
Jurassic period, 502-520 
close of, 505 
Jurassic system, 503 
thickness of, 505 
Jurassic of Arctic lands, 507 


Kansan glacial stage, 633, 634 

Kaolin, 257, 266 

Kames, 165, 631 

Kame terraces, 631 

Karroo beds, 470 

Kaskaskia series, 426 

Kayser, E., cited, 457 

Keewatin ice sheet, 613 

Kenai series, 559 

Kettles, 629 

Keuper formation, 491 

Keweenawan period, 331, 335 

Keweenawan rocks, 331, 334, 335 

Kinderhook fauna, 432, 433 

Kinderhook formation, 426 

King, C., cited, 606 

King, F. H., cited, 33 

Kingston beds, 402 

Kittatinny Mountain, 105 

Kittatinny peneplain, 104 

Knobs, 628, 629 

Knowlton, F. H., cited, 539. 561, 574 

Knox dolomite, 369 

Knoxville series, 525 

Kootenay formation, 525, 526 

Krakatoa, dust from, 13 
eruptions of, 237 

Kiimmel, H. B., cited, 203, 487, 523 

Kutorgina cingulata, 360 


Labradorean ice sheet, 613 
Labradorite, 254 
Labyrinthodonts, 440, 465 
Laccoliths, 228 
Lacustrine deposits, 202, 652 
Lafayette formation, 600, 602, 653 
Lakes, 201-205 

changes in, 201 


698 INDEX 


Lake Agassiz, 640, 641 
Algonquin, 639 
Arkona, 638 
basins, origin of, 205 
Bonneville, 203, 656, 657, 658 
Champlain, 639, 641 
Chicago, 636, 637 
cliffs, 179 
deposits, glacial, 660 
Duluth, 637 
ice, 121 
Troquois, 639 
Lahontan, 658 
Maumee, 636, 637 
Michigan, history of, 636 


Superior region, Proterozoic history, 


338 
Proterozoic rocks, 331 

Warren, 638, 639 

Whittlesey, 638 
Laminz, 269 
Lance formation, 539 
Land animals, Quaternary, 666 
Land snails, 467 
Landslides, 40 
Laplacian hypothesis. 

hypothesis. 
Laramie coal, 538 
Laramie series, 535, 538 
Lariosaurus balsami, 496 
Lateral moraines, 136, 156, 160 
Lateral movements, crustal, 223 
Lateral planation, 68 
Lava, 235, 250 

crystallization of, 251 

nature of, 238 

rise of, 226, 242, 244, 245 

source of, 240 

temperature of, 240 
Lava cones, 236 
Lava flows of the Deccan, 229 

of Iceland, 229 

of Lake Superior, 229 

of the northwest, 230 
Lassen Peak, 232 
Lawson, A. C., cited, 604, 663 
Lead and zinc of Missouri, 427 
Lead coral, 400 
Lead ores, Ordovician, 374 
Leadville limestone, 429 
Lecanocrinus macropetalus, 395 
LeConte, J., cited, 226, 604, 606 
Leda concentrica, 595 

glabra, 531 

parilis, 573 


See nebular 


Lee, W. T., cited, 530 
Leith, C. K., cited, 457 32100ge5 9945 
334, 337 
Lepadocystis moorei, 384 
Leperditia dermatoides, 359 
‘Lepidodendrons, 424, 459, 461, 462, 
476 
Lepidodiscus cincinnatiensis, 384 
Lepocrinites gebhardii, 414 
Laptena rhomboidalis, 382, 414, 435 
Lepterpeton dobbsi, 465 
Leptopora placenta, 433 
Leucophyres, 262 
Levees, 112 
Leverett, F., cited, 628, 632, 634, 645, 
676 
Life, beginning of, 312 
Life and glacial periods, 663 
Life of interglacial epochs, 664 
Lima wacoensis, 530 
Limestone, 194, 267, 268 
Limestone sinks, 39 
Limonite, 255, 256 
Lincoln, D. F., cited, 628 
Lindgren, W., cited, 561, 586 
Lingula brevirostra, 512 
rectilateralis, 382 
umbonata, 469 
Lingulepis pinniformis, 360 
Liriodendron giganteum, 547 
Lithosphere, 5 
composition of, 7 
Lithic era, 307 
Littoral currents, 175, 186 
Littoral deposits, 189 
Livingston formation, 539 
Lizards, Cretaceous, 549 
Llamas, 666 
Lockport limestone, 388 
Loess, 13, 14, 642, 645 
accessories, 645 
age of, 643, 644 
origin, 645 
thickness, 645 
Loess fossils, 644 
Lophospira helicteres, 381 
Lorraine beds, 368 
Loup Fork formation, 586, 592 
Lower Cambrian, 344 
Lower Carboniferous. 
sippian 
Lower Carboniferous of Europe, 430 
Lower Cretaceous of Europe, 527 
Lower Huronian, 331 
Lower Magnesian limestone, 369 


See Missis- 


INDEX 


Lowville limestone, 368 

Loxonema hamiltoniz, 419 
leda, 397 

Lucina aquiana, 573 

Lunatia marylandica, 573 

Lung-fishes, 422 

Lycopods, 458, 461 

Lyell, Sir Charles, cited, 676 

Lyginodendron oldhamia, 463 

Lyrodesma cincinnatiensis, 381 

Lytoceras batesii, 531 


Maclurea logani, 381 
Macrocheilus blairi, 433 
Magmatic segregation, 42 
Magmatic waters, 42 
Magnesium salts, 196 
Magnetite, 255, 256 
Magnolia pseudoacuminata, 547 
Malaspina Glacier, 129 
Mammals, Cretaceous, 550 

Eocene, 565, 566 

Miocene, 591 

Oligocene, 578 

Pliocene, 608 

Quaternary, 666 

Triassic, 496 
Mammoth cave, 30 
Man, antiquity of, 670-676 

a geological agent, 682 

relics of, 670 
Manganic nodules, 197 
Manlius limestone, 388 
Mantle rock, 7 
Maquoketa shale, 369 
Marble, 292 
Marcellus shale, 402, 404 
Marine deposits, 652 
Marine Quaternary fossils, 665 
Marl, 202 
Marquette iron ore, 333 
Mass action, 47 
Mastodon americanus, 667 
Matawan formation, 534 
Matthew, W. D., cited, 575, 593, 594 
Matteo Tepee, 94 
Mature topography, 71 

valleys, 71 
Maturity of shoreline, 182 
Mauch Chunk formation, 426, 427 
Mauna Loa, 231, 234 
McConnell, R. G., cited, 542 
McGee, W. J., cited, 600, 603, 645, 676 
Meanders of streams, 113, 514 
Meanders, entrenched, 102 


699 


Medial moraines, 136, 155, 156, I59 
Medlicottia capei, 481 
Medina sandstone, 388 
Meekella striatocostata, 469 
Meekoceras, 500 
Meekospira peracuta, 469 
Melaphyres, 262 
Men of Spy, 611 
Merced River, fan of, 110 
Merced series, 604, 607 
Mesabi iron ore, 332, 333 
Mesas, 93 
Mesohippus, 578 
Mesozoic era, 484 
Mesozoic, close of, 541, 542 
Metamorphic rocks, 9, 10 
Metamorphism, 285 
Metamorphism, dynamic, 291, 293 
thermal, 228, 201 
Metamorphism of Archean rock, 318 
of Proterozoic rocks, 339 
Meteorites, 3 
Meteoritic hypetheses, 200, 302 
Mica, 253, 255 
Mica schist, 292, 296 
Michelinia lenticularis, 414 
Michigan, copper in, 337 
Middle Cambrian, 345, 346 
Middle Huronian, 331 
Migration of dunes, 19 
Migration of life, in glacial period, 
664 
Millstone Grit, 441, 456 
Milne, J., cited, 206 
Minerals, formation of, 252 
Minnehaha Falls, 90 
Minnesota, Prcterozoic rocks of, 340 
Minor movements, 219 
Miocene climate, 591, 598 
Miocene, close of, 587 
faunas, provincial, 596 
igneous rocks, 587 
life, sor 
mammals, 5901 
period, 581-508 
system, distribution of, 582 
Mississippi, delta of, 115 
Mississippi River, erosion by, 84 
Mississippian fossils, 437 
life, 432 
Mississippian period, 426-440 
climate of, 432 
close of, 430 
Mississippian plants, 440 
system, 428, 430 











700 


Missouri, lead and zinc in, 374, 427 
zinc in, 427 
Missouri River, scour and fill by, 
113 
Missourian series, 443 
Mitra potomacensis, 573 
Modiolus alabamensis, 573 
Modiolus dalli, 595 
Mohawkian epoch, 368 
Molluscs, Cambrian, 361 
Cretaceous, 553-4 
Devonian, 418 
Eocene, 573 
Jurassic, 509 
Mississippian, 438 
Ordovician, 379 
Pennsylvanian, 469 
Silurian, 397 
Triassic, 498 
Monadnock, 94, 104 
Monmouth formation, 534 
Monocline, 276 
Monoclinal structure, 278 
Monongahela series, 443 
Monopteria longispina, 469 
Monotremes, 571 
Montana series, 535, 537 
Monterey formation, 583, 587, 607 
Monument Creek formation, 539 
Moon, mass of, 3 
Moonstone, 254 
Moraines on ice, 136 
Moraines, types of, 159 
Morrison beds, 504, 515, 519, 525 
Mosasaurians, 551 
Moulins, 138 
Mount Mitchell, 104 
Mountain-folding, 220 
Mountain limestone, 431 
Movements of crust, extent of, 223, 
224 
Movements of sea-water, 173 
Movements, recent, 677 
Muensteroceras oweni, 433 
Murray, Sir John, cited, 56, 189, 197, 
199 
Muscovite, 255 
Musk-ox, 666, 679 
Myacites humboldtensis, 500 
Myalina permiana, 481 
recurvirostris, 469 
Myriapods, 467 
Myrica longa, 547 
Myopharia alta, se9 
Mytilus whitei, 542 


INDEX 


Narragansett Bay coal measures, 445 
Narrows, 92 
Nassa marylandica, 597 
Naticopsis altonensis, 469 
Natural bridges, tor 
Natural bridge of Virginia, 102 
Natural levees, 112 
Nautilus meekanus, 554 
Neanderthal man, 611 
Nebular hypothesis, 300, 307 
earth under, 307 
objections to, 301, 308, 309 
Neocene, 556 
Neolithic age, 670 
Neumayria henryi, 512 
Neptunella intertextus, 554 
Neuropteris angustifolia, 460 
auriculata, 460 
valida, 478 
vermicularis, 460 
Neuropters, 467 
Névé, 126 
Newark series, 484, 486, 487, 488 
Newfoundland ice sheet, 615 
New River, 105 
New Scotland beds, 402 
Niagara Falls, 90, ot 
Niagara formation, 389, 390 
Niagara gorge, age of, 646, 647 
Niagara River, 79 
Niagaran coral reefs, 391 
epoch, 388 
series, 380 
Nickel in Huronian rocks, 334, 337 
Nile delta, 116 
Nipissing Lakes, 639 
Nodosaria bacillum, 572 
communis, 572 
Noeggerathiopsis hislop, 478 
Non-clastic sediments, 267 
North America, elevation of, 84 
Northern Interior coal-field, 444 
Nova Scotia coal-field, 445 
Nucleocrinus verneuili, 416 
Nyctosaurus gracilis, 550 
Nucula ovula, 574 
storrsi, 531 


Obsidian, 249, 258, 262 
Ocean, age of, 168 
area of, 5 
beginning of, 311 
depth of, 4 
work of, 167-200 
Ocean basins, 5, 168 


Ocean basins,— continued 
topography of, 6, 168 
Ocean life, 169 
Ocean water, 167 
movements of, 173 
Oceanic era, 307 
Odontocephalus egeria, 416 
Odontopteris cornuta, 460 
Oenonites rostratus, 386 
Ohio gas and oil, 374 
Oil and gas, Devonian, 410 
Miocene, 583 
Ordovician, 374 
Pennsylvanian, 454 
Old age, topographic, 72 
Old Red Sandstone, 411, 421 
Oldest rocks, 316 
Olenellus fauna, 348 
Olenellus gilberti, 348 
Olenoides curticei, 359 
Olenoides fauna, 349 
Oligocene, marine life, 580 
Oligocene of Europe, 576 
Oligocene period, 556, 574-580 
plants, 578 
Oligoporus mutatus, 434 
Oliva litterata, 597 
Olivine, 255 
Oncoceras pandion, 380 
Oneida conglomerate, 388 
Onondaga fauna, 415, 416 
limestone, 402, 404, 405 
Onyx, 254 
Odlite, 289 
Ooze, 197, 268 
Ophileta primordalis, 361 
Orange sand, 600 
Ordovician igneous rocks, 370 
Ordovician in Europe, 375 
Ordovician land life, 387 
Ordovician pence 367-387 
climate of, 376 
close of, 373 
duration of, 376 
life, 376 
plants, 386 


Ordovician rocks, changes in, 371 


outcrops of, 371, 372 
thickness of, 371 
Ordovician, sections of, 368 
Ordovician sedimentation, 367 
Ore, 40 
Ore deposits, 40 
Ore regions, origin of, 44 
Ores, concentration of, 44 


INDEX 


Ores,— continued 
location of, 46 
Organic deposits, 193, 197, 198 
terrestrial, 652 
Original crust, 308 
Oriskany fauna, 415 
Oriskanian series, 402, 404 
Orogenic movements, 218 
Cretaceous, 541 
Jurassic, 505 
Permian, 475 
Orohippus, 569 
Orthis tricenaria, 382 
Orthoceras annulato-costatum, 427 
annulatum, 397 
bilineatum, 380 
blakei, 500 
Cretaceous, 555 
cribrosum, 469 
rushensis, 481 
sociale, 380 
Orthoclase, 254 
Orthopters, 467 
Orton, E., cited, 374 
Osage fauna, 433, 435 
Osage series, 426, 427 
Osborn, H. F., cited, 567 
Ostracoderms,’421, 440 
Ostrea carolinensis, 595 
compressirostra, 573 
deltoidea, 510 
soleniscus, 554 
strigilecula, 512 
Oswegan epoch, 388, 380 
Ouachita Uplift, 373 
Oudenodon trigoniceps, 495 
Outcrops, width of, 353, 354 
Outwash plains, 165 
Ox-bow lakes, 115 
Oxidation, 23 


Palzaster simplex, 384 
Paleocaris typus, 466 
Paleohatteria longicaudata, 479 
Paleoneile constricta, 419 
Palzophonus caledonicus, 407 
Palzospondylus gunni, 422 
Paleolithic age, 670 
Paleozoic era, close of, 475 
Paradoxides, 348 

bohemicus, 348 
Paralegoceras newsomi, 469 
Paraphorynchus striatocostatus, 433 
Parasmilia texana, 530 
Pareiasaurus, 478 


701 


702 | INDEX 


Pareiasaurus serridens, 480 Perlites, 263 
Paso Robles formation, 604 Permanent streams, 538 
Patagonian beds, 590 origin of, 61 
Patrician ice sheet, 613 Permian amphibians, 477 
Patriofelis, 570 aridity, 470 
Peale, A. C., cited, 504 climate, 476 
Pecopteris unita, 460 fossils, 481 
Pecten choctawensis, 574 glacial formations, 474 
complexicosta, 531 life, 475 
humboldtensis, 500 Permian period, 470-483 
madisonius, 595 plants, 476 
texanus, 530 problems, 482 
Pedinopsis pondi, 554 reptiles, 477 
Pegmatite, 260 Permian, thickness of, 471 
Pelagic deposits, 189, 197, 198 Pernopecten cooperensis, 433 
Pelecypods, 500, 512 Petrified trees, 589 
Cambrian, 361 Petrified wood, 35, 287 
Cretaceous, 554 Phacoides foremani, 595 
Devonian, 418 Phacops logani, 414 
Jurassic, 509, 510 rana, 419 
Miocene, 595 Phanerites, 248, 259, 262 
Mississippian, 432 Phenacodus primevus, 567 
Ordovician, 380, 381 Phillipsia major, 469 
Permian, 480, 481 Phinney, A. J., cited, 374 
Silurian, 397 Phobos, 301 
Pelée, 238 Phosphates, Devonian, 411 
Pelycosaurus, 477 Ordovician, 374 
Peneplain, 66, 67, 72 Phosphatic nodules, 197 
Penokee-Gogebic iron ore, 332, 333 Phragmoceras nestor, 397 
Penrose, R. A. F., cited, 44, 583 Phylloceras knoxvillensis, 531 
Pennsylvanian fauna, 464 Phyllograptus (?) cambrensis, 362 
flora, 458 ilicifolius, 385 
Pennsylvanian, foreign, 457 typus, 385 
Pennsylvanian insects, 467 Phyllotheca indica, 478 
Pennsylvanian in West, 445 Piedmont alluvial plain, r10 
Pennsylvanian iron ore, 454 Piedmont glaciers, 129 
land life, 466 Pilot Rock, 616 
marine fauna, 469 Pinnipeds, 571 
oil and gas, 454 Piracy, 65, 97, 98 
Pennsylvanian period, 441-469 Pitchstone, 258, 263 
close of, 456 Pithecanthropus erectus, 610, 611 
duration of, 455 Plagioclase, 254 
Pennsylvanian sea life, 468 Planetesimal hypothesis, 300, 303, 310 
system, 442 Planets, 2 
thickness, 447 Plants, classification of, 685 
Pentacrinus briareus, 511 Platacarpus corypheus, 550 
Pentamerus oblongus, 396 Plateau-forming movements, 220 
Pentremites robustus, 436 Platephemera antiqua, 424 
Perched bowlders, 163 Platyceras dumosum, 416 
Peridot, 255 gibbosum, 414 
Peridotites, 260, 262 primzvum, 361 
Periodic movements, 219, 220 Platyostoma niagarensis, 397 
Perisphinctes tiziani, 508 broadheadi, 433 
Perissodactyls, 568 Platystrophia lynx, 382 


Peristerite, 254 Plectambonites sericeus, 382 


Pleistocene glaciers in western moun- 


tains, 657 

life, 663 

period, 613-676 
Plesiosaurs, 514 

Cretaceous, 550 
Plesiosaurus dolichodeirus, 514 
Pleurocystis filitextus, 384 
Pleurotoma potomacensis, 573 

tysoni, 573 
Pleurotomaria nodulostriata, 437 
Pliocene diastrophism, 604 

formations, marine, 604 

life, 608, 611 

of Europe, 608 

period, 599-612 

plants, 608 

volcanic activity, 607 © 
Pliohippus, 592 
Plutonic rocks, 227, 245 
Pocono formation, 426 
Polar glaciers, 127 
Polygyra clausa, 644 

monodon, 644 

multilineata, 644 
Polynices duplicatus, 597 
Polypora lilea, 414 
Ponding of streams, 107 
Popanoceras walcotti, 481 
Porphyry, 248, 251, 259, 262 
Porphyritic texture, 249 
Portage beds, 402 
Poterioceras apertum, 380 
Potomac River, 105 
Potomac series, 523 
Potsdam epoch, 344, 346 
Pottsville Conglomerate, 441. 443 
Prase, 254 
Precipitation, 28 
Precipitation from solution, 24 
Prestwichia dane, 466 
Primates, 571 

Miocene, 594 

Pliocene, 610 
Prinotropis woolgari, 554 
Proboscidians in America, 667 
Proboscidians, Pliocene, 610 
Prodromites gorbyi, 433 
Productella pyxidata, 433 

spinulicosta, 416 
Productive coal-fields, 443 
Productus, 436, 438 

arcuatus, 433 

burlingtonensis, 435 

costatus, 469 


INDEX 703 


Productus — continued - 

’ fasciculatus, 437 
marginicinctus, 437 
nebrascensis, 469 
symmetricus, 469 

Proétus ellipticus, 433 
parviusculus, 378 

Profiles of valleys, 112 

Progonoblattina columbiana, 466 

Prosser, C. S., cited, 443 

Proterozoic climate, 342 

Proterozoic era, 325 
duration of, 337 

Proterozoic glacial formations, 342 

Proterozoic life, 342 

Proterozoic rocks, composition of, 325 

327, 329 . 
distribution of, 328, 331, 341 
origin of, 327 
relations to Archean, 325, 33¢ 
structure of, 329 

Proterozoic, subdivisions of, 326 

Protocardia levis, 573 

Protohippus, 592 

Protospongia maculosa, 386 

Protowarthia cancellata, 381 

Protozoans, Pennsylvanian, 468 

Pseudomonotis curta, 512 
hawni, 481 

Pseudomorph, 258, 287 


| Psychozoic era, 683 


Pteridophytes, 458 
Pteridosperms, 458 
Pterinea demissa, 387 
emacerata, 397 
flabella, 419 
Pterodactyls, 5190 
Cretaceous, 550 
Pterodactylus spectabilis, 519 
Pteropod ooze, 199 
Pterosaurs, 518 
Pterygometopus callicephalus, 378 
Pterygotus, 400 
anglicus, 401 
Ptychoceras crassum, 554 
Ptycoparia kingi, 359 
Puget series, 559 
Puget Sound depression, 559 
Pugnax uta, 460 
Pulpit-rocks, 179, 181 
Pumice, 250, 258, 263 
Pupa vermilionensis, 466 
Pyrite, 258 
Pyroclastic rocks, 229, 235, 250 
Pyropsis bairdi, 554 


704 


Pyroxene, 253, 255 
Pyroxenite, 260, 262 


Quartz, 253, 254 
Quartzite, 292 
Quaternary, 556 
eruptions, 661 
life, 663 
marine deposits, 661 
non-glacial formations, 651 
Quercus suspecta, 547 


Radiolarian ooze, 199 
Rafinesquina alternata, 382 
Rain, effects of, 28 
Raindrop impressions, 272 
Raised beaches, 662 
Rancocas formation, 534 
Ransome, F. L., cited, 408 
Raphistomina lapicida, 381 
Rapids, 89 
Raton formation, 539 
Ravines, 58 
Recent formations, 677 
Recent movements, 677 
Receptaculites occidentalis, 386 
oweni, 400 
Recessional moraine, 631 
Reconstructed glaciers, 130 
Red Beds, 445, 470, 489 
Red clay of deep sea, 199 
Red mud, 198 
Red Sea, 608 
Reid, H. F., cited, 131, 133, 134 
Reindeer, 679 
Reineckia brancoi, 508 


Rejuvenation of streams, Io1, 103 


Relief, 18 
Rensselzria equiradiata, 414 
ovoides, 415 
Replacements, 286 
Reptiles, Cretaceous, 546 
Eocene, 572 
Jurassic, 513 
Permian, 477 
Triassic, 494 
Residuary ores, 44 
Reteograptus eucharis, 385 
Reticularia pseudolineata, 435 
Retusa conulus, 597 
Rhetic formation, 491 


Rhamphorhynchus phyllurus, 518 


Rhinidictya mutabilis, 383 
Rhinoceros, 579 


INDEX 


Rhipidomella burlingtonensis, 435 
oblata, 414 
vanuxemi, 419 
Rhynchonella equiplicata, 500 
gnathophora, 512 
whitneyi, 531 
Rhynchotrema capax, 382 
Rhytimya radiata, 381 
Richmond beds, 368, 388 
Rill-marks, 192 
Ripley formation, 535 
Ripple-marks, 21, 191, 471 
River ice, 122 
Roan Mountain, 104 
Roanoke River, .105 
Roches moutonnées, 161, 162, 622 
Rochester shale, 388 
Rock breaking by changes of tempera- 
ture, 25 
Rock-salt, 268 
Rock structure and erosion, 81 
Rock terraces, 91 
Rock waste, 265, 285 
Rocks, varieties of, 8, 246, 258, 267, 292, 


207 
Rocky Mountains, development of, 541 
Rodents, Eocene, 571 
Miocene, 594 
Rondout waterlime, 388 
Roots, and rock breaking, 74 
Ropy lava, 239 
Rose quartz, 254 
Rothliegende, 472 
Running water, work of, 56-120 
Russell, I. C., cited, 40, 107, 31, 140, 
202, 203, 245, 628, 658 


St. Anthony Falls, 90 
age of, 647 
St. Croixan epoch, 344, 346 
St. Louis faunas, 436 
series, 426, 427 
St. Peter sandstone, 369 
Safford, J. M., cited, 600 
Salem formation, 427 
Salenia tumidula, 554 
Salina beds, 388, 392 
Salisbury, R. D., cited, 131, 203, 226, 
366, 376, 600, 628, 630, 645 
Salt, Silurian, 391 
Salt in New York, 392 
in Ohio, 392 
Salt deposits, 195 
Salt lakes, 203 
Salton Sink, 118 


Salts of sea, 167 
Sandstone, 267, 270 
Sangamon interglacial stage, 635 
Sangamon-Peorian epoch, 633 
Santa Cruz beds, 590 
Sapping, 89 
Saratogan epoch, 344, 346 
Sassafras subintegrifolium, 547 
Satin spar, 257 
Saurians, Cretaceous, 546 
Jurassic, 515 
Triassic, 404 
Savage, T. E., cited, 368 
Scala potomacensis, 573 
sayana, 507 
Scaphites nodosus, 554 
Schist, 295, 297 
Schistosity, 10, 294, 295 
Schizocrania filosa, 382 
Schizodus chesterensis, 437 
Schizolopha textilis, 381 
Schizoneura gondwanensis, 478 
Schizophoria multistriata, 414 
swallovi, 435 
Schizotreta ovalis, 382 
Schoharie grit, 402 
Schuchert, Chas., cited, 417 
Schwarz, E. H. 1.., cited, 413 
Scoriaceous rock, 250 
Scorpions, Silurian, gor 
Scott, E. H., cited, 461 
Scott, W. B., cited, 365, 567 
Scour-and-fill, r12 
Sea. See ocean. 
Sea caves, 180, 181 
cliffs, 178 
Sea coast, simplification of, 182 
Secular movements, 217 
Sedgwickia topekaensis, 481 
Sediment, cementation of, 267 
deposition of, 266 
how carried, 76 
Sedimentary rocks, 265 
Sedimentation, Cambrian, 351 
variations in, 351 
Sediments, induration of, 285 
Seed-bearing ferns, 423 
Seiches, 201 
Seismology, 206 
Selenite, 257 
Selma Chalk, 535 
Seminula argentea, 469 
subquadrata, 437 
Senecan series, 402, 406 
Septaria, 289 


INDEX 795 


Septarian nodule, 290 
Serpentine, 258, 291 
Sevier shale, 369 
Seward, A. C., cited, 461 
Shale, 267, 270 
Shaler, N. S., cited, 39, 179 
Shallow-water deposits, 189, 190, 193 
characteristics of, 192 
Sharks, Mississippian, 438 
Shastan fossils, 531 
Shastan group, 525 
Shearing of ice, 143 
Sheet erosion, 59 
Shimek, B., cited, 645 
Shore-currents, 175, 183 
deposition by, 184 
Shore-deposition and shore lines, 187 
Shore drift, 183 
Shore lines, 170 
and deposition, 187 
mature, 182 
warping of, 662 
Short hills, 628 
Shrinkage of earth, 225, 226 
Sierra Nevada, history of, 606 
peneplanation of, 585 
Sigillarias, 459, 461, 462, 476 
Siliceous deposits, 268 
Siliceous sinter, 660 
Silurian of West, 393 
Silurian period, 388-401 
climate of, 393 
close of, 393 
duration of, 393 
life of, 394 
Silver in Huronian rocks, 334, 337 
Siphonalia marylandica, 597 
Sirenians, 571 
Slate, 294, 393 
Slichter, C. S., cited, 33, 34 
Slickensides, 283 
Slumps, 40 
Smith, E. A., cited, 600 
Smith, G. O., cited, 607 
Smith, J. P., cited, 564 
Snakes, Cretaceous, 549 
Snow and ice, work of, 121-166 
Snow-fields, 124 
Snowflakes, 141 
Snow-line, 125 
Soapstone, 291 
Soil-fertility, 680 
Solar system, earth in, 2 
evolution of, 301, 305 
Solarium trilineatum, 597 


706 


Solution of mineral matter, 35 
importance of, 36 
South Africa, glacial Permian in, 474 
South America, coal in, 457 
Cretaceous in, 545 
Devonian, 413 
Eocene in, 564 
glacial Permian in, 474 
Jurassic in, 507 
Miocene of, 590 
Triassic in, 493 
South Dakota, Proterozoic rocks of, 
_ 340 
»patter-cones, 237 
Specific gravity of earth, 11 
Spencer, J. W., cited, 605, 646 
Spergen formation, 427 
Spherexochus mirus, 399 
Sphenophylls, 458, 461 
Sphenophyllum longifolium, 461 
Sphenopteris splendens, 460 
Spiders, 467 
Spiral nebula, 303 
Spiral nebulz, composition of, 304 
evolution of, 305 
organization of, 304 
Spirifer acuminatus, 416 
arenosus, 415 
biplicatus, 433 
cameratus, 469 
increbescens, 437 
logani, 435 
macropleurus, 414 
marionensis, 433 
murchisoni, 415 
niagarensis, 396 
pennatus, 419 
radiatus, 396 
striatus, 435 
suborbicularis, 435 
Spiriferina kentuckiensis, 469 
spinosa, 437 
Spirifers, 418 
Spisula marylandica, 595 
Spit, 186 
Sponges, Cambrian, 362 
Ordovician, 385, 386 
Silurian,.300.. 5° 
Spouting horns, 181 
Springs, 48 
Squaloraja polyspondyla, 513 
Squamata, 551 
Stalactites, 287 
Stalagmites, 287 
Stanton, T. W., cited, 531, 530 


{INDEX 


Starfish, Silurian, 396 
Staurocephalus murchisoni, 399 
Steatite, 201 
Stegocephalians, 465 
Stegosaurs, 517, 518 
Stenotheca rugosa, 361 
Sterculia mucronata, 547 
Stone, G. H., cited, 642, 666 
Stratification, 269 
Stratified rock, 9, 269 
Stratum, 269 
Stream erosion, 59, 73-80 
Streams, intermittent, 58 
permanent, 58 
Streptelasma corniculum, 385 
Streptis grayl, 396 
Strike, 275 
Strobilops labyrinthica, 644 
Stromatopora delicatula, 383 
Stropheodonta concava, 416 
magnifica, 415 
profunda, 396 
Strophomena subtenta, 382 
Strophonella punctulifera, 414 
Structural valleys, 63 : 
Subaérial formations, Pliocene, 59g 
Sub-Aftonian ice sheet, 633 
Subglacial drift, 147, 153 
Submerged valleys, 171 
Submergence, causes of, 349 
Subulites regularis, 381 
ventricocus, 397 
Succinea avara, 644 
obliqua, 644 
Suina, 570 
Sun-cracks, 192, 194, 195 
Sunflower coral, 400 
Sunstone, 254 
Superglacial drift, 147, 153 
Superimposed drainage, 99 
Surcula biscatenaria, 597 
Susquehanna River, 105 
Syenites, 260, 262 
Synclines, 276 
Synclinoria, 277 
Syringopora verticillata, 398 


Tabular joints, 280 

Taconic Mountains, 371 
age of, 373 . 

Taff, J. A., cited; 432 

Talc, 258 

Talchir conglomerate, 474 

Talcose rocks, 291 


Talus, 27, 74) 75 


INDEX 


Tapir, 666 
Tarr, R., cited, 103, 632 
Taylor, F. B. , cited, 646 
Teleosts, 512 
Tellina producta, 595 
Temnocheilus forbesianus, 469 
Temperature changes, effect on rocks, 25 
Temperature, increase with depth, 225 
Temperature of sea water, 169 
Tennessee phosphates, 374 
Terebra unilineata, 597 
Terebratula deformis, 500 
Terminal moraines, 155, 163, 628 
topography of, 630 
Terrigenous deposits, 189, 197 
Terraces, II19, 120 
wave-built, 187 
wave-cut, 181 
Tertiary, 556-612 
Tertiary lava flows, 229, 561, 587 
Tetrabelodon angustidens, 591 
Tetragraptus bigsbyi, 385 
fruticosus, 385 
Textularia subangulata, 572 
Thalattosaurs, 501 
Thames River, solution by, 36 
Thamnastrea prolifera, 511 
Thecosmilia trichotoma, 511 
Thermal metamorphism, 291 
Theropoda, 495 
Thorcfares, 187 
Tight, W. G., cited, 632 
Till, 618, 627 
Tillotherium fodiens, 571 
Time divisions of geology, 323 
Titanotheres, 570 
Titanotherium validum, 578 
Todd, J. E., cited, 632 
Topographic effects of glacial erosion, 
I51 
Topographic maps, 17 
Topographic youth, 71 
Tornatellza bella, 573 
Tornoceras mithrax, 416 
Toronto interglacial beds, 665 
Trachyceras austriacum, 408 
Transportation by streams, 73, 76 
Transportation by waves, 183 
Trap. 761 
Travertine, 287 
Tremataspis, 422 
Trematis mailepunctata, 382 
Trenton Falls, 369 
Trenton limestone, 368, 369 
Triassic fossils, 500 


Triassic life, 494 
Triassic period, 484-5o0r 
climate of, 490 
close of, 490 
igneous rocks of, 487 
Tributaries, development of, 54 
Triceratops prorsus, 548 
Trigonia emoryi, 530 
navis, 510 
Trilobites, Cambrian, 348, 350 
Devonian, 414, 417 
Ordovician, 378, 379 
Silurian, 399 
Trimerella acuminata, 306 
ohioensis, 396 
Trinity formation, 524 
Trinucleus ornatus, 378 
Trocholites ammonius, 380 
Trocus saratogensis, 361 
Troostocrinus reinwardti, 395 
Tropidoleptus carinatus, 419 
Tropites subbullatus, 4098 
Truncatulina lobatula, 572 
Tufa deposits, 204 
Tuffs, 250, 258 
Tully limestone, 402 
Turbo moyonensis, 531 
Turner, H. W., cited, 586 
Turritella budaensis, 530 
mortonl, 573 
variabilis, 597 
Turtles, Cretaceous, 540, 551 
Jurassic, 519 
Tuscaloosa series, 523 
Tyrrell, J. B., cited, 613, 628 


Udden, J. A., cited, 12 

Ulrich, E. O., cited, 367, 406 
Ulsterian series, 402 

Uncinulus mutabilis, 414 
Unconformities, 272 

Undertow, 183, 184 

Undina gulo, 512 

Unequal hardness, effects of, 80 
Unio douglassi, 530 

Unio farri, 530 


707 


Upham, Warren, cited, 203, 628, 646 


Upper Cambrian, 346, 347 
Upper Carboniferous period, 441 
Upper Huronian, 331 

Utica shales, 368 


Vaginalina legumen, 572 
Valley glaciers, 127 
Valley trains, 164 


708 


Valleys, 58 
development of, 60 
limits of, 63 


Van Hise, C. R., cited, 45, 226, 321, 325, 
331, 334, 3 

Veatch, A. é& fee 538 

Veins, 286 


Venaricardia marylandica, 573 
Venus ducatelli, 595 
Vermes, Cambrian, 361, 362 

Ordovician, 385, 386 
Vermilion iron ore, 333 
Vesuvius, 230 
Viburnum inequilaterale, 547 
Vicksburg formation, 574 
Vitulina pustulosa, 419 
Viviparus montanensis, 530 
Volcanic activity, Pliocene, 607 
Volcanic agglomerate, 250, 263 

ash; 235,250 

breccia, 263 

cones, 236 

conglomerate, 263 

dust, 13 

gases, 236, 241 

mud-flows, 238 

plugs, 228 

rocks, 227, 246, 652 
Volcanic stages of earth’s history,311,312 
Volcanoes, 230 

distribution of, 231, 233 

gases of, 236, 240, 241 

independence of, 232 

periodicity of, 234 

products of, 235 

relations of, 232 
Voltzia heterophylla, 478 
Vulcanism, 2, 237-245 

cause of, 241 
Vulcanism in sea, 172 


Waagenoceras cumminsi, 481 
Walchia piniformis, 477 
Walcott, C. D., cited, 342 
Walled lakes, 122 
Washington, coal in, 559 
Water-gaps, 92, 104 
Water-table, 31 
Waterlime, fauna of, gor 
Wave-built terraces, 187 
Wave-cut terraces, 181 
Wave erosion, 181 
motion, 173 
Waverly fauna, 435 
series, 427 


INDEX 


Waves, 173, 175 
deposition by, 184 
erosion by, 176, 177 
transportation by, 183 
Weathering, 29, 38, 73, 74 
Weed, W. H., cited, 37, 51 
Weller, S., cited, 368 
West Indian Eocene, 564 
White, David, cited, 423, 454 
White, leGe cited, 440, 453; 457 
White River beds, 575, 578 
fauna, 591 
Whitney, J. D., cited, 676 
Wichita formation, 471, 524 
Wild, J. J., cited, 167 
Wilder, F. A., cited, 560 
Williams, H. S., cited, 358 
Willis, Bailey, cited, 96, 177, 226, 357. 
542, 559, 588, 607, 645 
Williston, S. W., cited, 464 
Wilson, J. H., cited, 613 
Winchell, N. H., cited, 646 
Wind, abrasion by, 22 
work of, 12 
Wind-gaps, 104 
Wind-ripples, 21 
Winslow, Arthur, cited, 374 
Wisconsin ice sheet, 633, 635 
Wisconsin, lead and zinc in, 374 
Wisconsin River, Dells of, 100 
Worms. See vermes. 
Worthenia tabulata, 469 
Wyandotte cave, 39 


Xenoneura antiquorum, 424 
conchyliophora, 597 


Yarmouth epoch, 633 

interglacial stage, 634 
Yellowstone falls, 89 
Yellowstone National Park, 589 
Youth of valleys, 69 


Zaphrentis centralis, 435 
ponderosa, 416 
umbonata, 398 
Zechstein, 472 
Zeolites, 197 
Zeuglodons, 571 
Zinc of Missouri, 427 
Zinc ores, Ordovician, 374 
Zone of fracture, depth of, 240 
Zonites priscus, 466 
Zonitoides minusculus, 644 
Zygospira recurvirostris, 382 








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